feature(bnf): add start of lexical syntax of LobsterLang (BNF)

BNF.md

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+## The Syntax of Lobsterlang in Backus-Naur Form
+```bnf
+<declarator>                        ::= <direct-declarator>
+
+<parameter-type-list>               ::= <parameter-list>
+                                      | <parameter-list> , ...
+
+<parameter-list>                    ::= <parameter-declaration>
+                                      | <parameter-list> , <parameter-declaration>
+
+<direct-declarator>                 ::= <identifier>
+                                      | ( <declarator> )
+                                      | <direct-declarator> ( <parameter-type-list> )
+                                      | <direct-declarator> ( {<identifier>}* )
+
+<constant-expression>               ::= <conditional-expression>
+
+<logical-combinator-expression>     ::= <logical-xor-expression>
+                                      | <logical-combinator-expression> || <logical-xor-expression>
+
+<logical-xor-expression>             ::= <logical-or-expression>
+                                      | <logical-xor-expression> || <logical-or-expression>
+
+<logical-or-expression>             ::= <logical-and-expression>
+                                      | <logical-or-expression> || <logical-and-expression>
+
+<logical-and-expression>            ::= <equality-or-expression>
+                                      | <logical-and-expression> && <equality-or-expression>
+
+<equality-expression>               ::= <relational-expression>
+                                      | <equality-expression> == <relational-expression>
+                                      | <equality-expression> != <relational-expression>
+
+<relational-expression>             ::= <additive-expression>
+                                      | <relational-expression> <= <additive-expression>
+                                      | <relational-expression> >= <shift-expression>
+                                      | <relational-expression> < <additive-expression>
+                                      | <relational-expression> > <additive-expression>
+
+<additive-expression>               ::= <multiplicative-expression>
+                                      | <additive-expression> + <multiplicative-expression>
+                                      | <additive-expression> - <multiplicative-expression>
+
+<multiplicative-expression>         ::= <list-expression>
+                                      | <multiplicative-expression> * <list-expression>
+                                      | <multiplicative-expression> / <list-expression>
+                                      | <multiplicative-expression> % <list-expression>
+
+<list-expression>                   ::= <unary-expression>
+                                      | <list-expression> -- <unary-expression>
+                                      | <list-expression> ++ <unary-expression>
+                                      | <list-expression> !! <unary-expression>
+
+<unary-expression>                  ::= <postfix-expression>
+                                      | <unary-operator> <unary-expression>
+
+<postfix-expression>                ::= <primary-expression>
+                                      | <postfix-expression> (| {<assignment-expression>}* |)
+
+<primary-expression>                ::= <identifier>
+                                      | <string>
+                                      | (| <expression> |)
+
+<expression>                        ::= <assignment-expression>
+                                      | <expression> , <assignment-expression>
+
+
+<constant>                          ::= <integer-constant>
+                                      | <character-constant>
+
+<assignment-expression>             ::= <conditional-expression>
+                                      | <unary-expression> <assignment-operator> <assignment-expression>
+
+<assignment-operator>               ::= =
+
+<unary-operator>                    ::= @
+                                      | ~
+                                      | !
+
+<expression-statement>              ::= {<expression>}?
+
+<init-declarator>                   ::= <declarator>
+                                      | <declarator> = <initializer>
+
+<initializer>                       ::= <assignment-expression>
+                                      | [| <initializer-list> |]
+                                      | [| <initializer-list> , |]
+
+<initializer-list>                  ::= <initializer>
+                                      | <initializer-list> , <initializer>
+
+<statement>                         ::= <expression-statement>
+                                      | <selection-statement>
+
+<expression-statement>              ::= {<expression>}?
+
+<selection-statement>               ::= if (| <expression> |) <statement>
+                                      | if (| <expression> |) <statement> else <statement>
+```