New documentation support

[i582]

Note: this PR is not ready for review yet, only for discussion of changes in support documentation in general.
Note2: until #199 is merged, this PR also contains code from it

In this PR we use the new documentation API: https://plugins.jetbrains.com/docs/intellij/documentation.html#documentation-target-api

This PR supports following symbols:

• Functions
  • Attributes
  • Public/Native/Entry modifiers
  • Type parameters
  • Acquires (WIP)
  • Highlight usage of type parameters in signature
• Modules
• Structs
  • Attributes
  • Type parameters
  • Abilities
• Named fields
  • Attributes
• Enums
  • Attributes
  • Type parameters
• Enum variants (both with block fields and positional)
  • Attributes
  • Documentation from comments
• Constants
  • Attributes
• Schemas
  • Type parameters
• Named addresses
  • Documentation from inline comments in TOML

This PR introduces color highlighting for all signatures:

Big things to do

• Tests
• Cleanup
• Documentation with ctrl-hover

Fixes #202

[mkurnikov]

I'm closing this PR, as #250 merged and covers the same scope.