Add builtin functions

[mikelu92]

This addresses #55

Checklist:

• [ ] All tests pass in CI.
• [ ] There are sufficient tests for the new fix/feature.
• [x] Grammar rules have not been renamed unless absolutely necessary.
• [x] The conflicts section hasn't grown too much.
• [x] The parser size hasn't grown too much (check the value of STATE_COUNT in src/parser.c).

[matoous]

@aryx can we close this as this was addressed by https://github.com/tree-sitter/tree-sitter-go/pull/96 ?

[aryx]

Sure