feat(grammar): Refactor modifiers for better querying

[lightless233]

Addresses #204.

Right now, modifiers like public or static are just anonymous keywords in the AST. This makes it a pain to write queries for them without matching on text.

This PR makes them actual nodes, similar to how tree-sitter-typescript handles it.

Changes:

1. New (visibility) node:

   • public, protected, private are now grouped under this node.
   • Makes it easy to find declarations with specific visibility.

   ; e.g., find public methods
   (method_declaration (modifiers (visibility) @v) (#eq? @v "public"))

2. New (modifier) node:

   • static, final, abstract, etc., now all produce a (modifier) node.
   • This provides a single node type for all these other keywords.

   ; e.g., find static fields
   (field_declaration (modifiers (modifier) @m) (#eq? @m "static"))

Resulting AST:

For public static class..., the AST is now much cleaner:

- (modifiers (KEYWORD) (KEYWORD))
+ (modifiers
+   (visibility)
+   (modifier)
+ )

Closes #204.