Tree Sitter Parser to Custom Types

[spreston8]

• parse_proc is main function and below it are a bunch of helper functions
• rholang_ast.rs contains custom types
• grammar.js is source of truth. In grammar.js, I've modified:
  1. Added var_ref
  2. Changed _parenthesized to parenthesized. I did this to make it easier to match against. If this is a breaking change and needs to be reverted then I can figure out another way to handle this case.
  3. Added mod to _proc_expression. It was missing from the choice and believe this was a mistake that I am correcting.

[marcin-rzeznicki]

3. Added `mod` to `_proc_expression`. It was missing from the choice and believe this was a mistake that I am correcting.

Yes, definitely, thanks for correcting