Add support for internal visibility modifier (#67)

Summary:
Modern Hacklang introduced modules and the internal visibility modifier
to mark methods and functions as only accessible from within the current
module.

The Hacklang treesitter grammar doesn't support the internal visibility
modifier yet. This diff addresses it.

Test Plan:

Grammar tests pass:
```
tree-sitter generate
tree-sitter test
```

Reviewers:

Subscribers:

Tasks:

Tags:

Author: andryak

grammar.js

@@ -1091,7 +1091,7 @@ const rules = {
 
   static_modifier: $ => 'static',
 
-  visibility_modifier: $ => choice('public', 'protected', 'private'),
+  visibility_modifier: $ => choice('public', 'protected', 'private', 'internal'),
 
   attribute_modifier: $ =>
     seq('<<', com($.qualified_identifier, opt($.arguments), ','), '>>'),