Add support for custom compilers in solc-typed-ast versions

[cd1m0]

This pr adds support for custom compiler paths in solc-typed-ast. To use a custom compiler you can specify a verison such as "custom:/":

sol-ast-compile --compiler-kind native --compiler-version 'custom:/path/to/solc' file.sol

[cd1m0]

lgtm

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 75.67568% with 9 lines in your changes missing coverage. Please review.

Project coverage is 93.72%. Comparing base (e92fc59) to head (a9c0045).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
src/compile/kinds/compiler.ts	85.71%	4 Missing ⚠️
src/compile/version.ts	57.14%	3 Missing ⚠️
src/types/infer.ts	0.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #273      +/-   ##
==========================================
+ Coverage   91.11%   93.72%   +2.60%     
==========================================
  Files         270      270              
  Lines        6503     6513      +10     
  Branches     1320     1322       +2     
==========================================
+ Hits         5925     6104     +179     
+ Misses        469      386      -83     
+ Partials      109       23      -86     

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