Fix pascal/camel case identifier tokens to allow underscores

[mallyskies]

Fixes two related lexing bugs in _pascalCaseIdentifier/_camelCaseIdentifier that cause any identifier shaped like Foo_Bar (or a leading-underscore type name) to be silently truncated or rejected wherever it's used as a type_name — extends/implements targets, new X(...) constructors, and import paths.

Truncated at the first underscore after a case shift

_pascalCaseIdentifier original trailing character class excludes _, so once the leading run of uppercase letters ends, an underscore stops the match. class Foo extends Bar_Baz {} parses Bar_Baz as just Bar, leaving _Baz as a separate ERROR node. Same issue in _camelCaseIdentifier for lowerCamel names with an underscore after the first case shift (e.g. fooBar_Baz).

Confirmed via direct parse — before this fix: class Foo extends Bar_Baz {}

produces type_name: 'Bar' followed by an ERROR node for _Baz.

Type names starting with _ were rejected outright

Per the Haxe manual: "Type names must start with an upper-case letter A-Z or an underscore _." _pascalCaseIdentifier's leading character class was [A-Z]+, which can never match a leading _, so a module-private type name (Haxe's _Foo convention) fails to lex as a type_name reference at all — not truncated, just unrecognized.

Fix

-    _camelCaseIdentifier: ($) => /[a-z_]+[a-zA-Z0-9]*/,
-    _pascalCaseIdentifier: ($) => /[A-Z]+[a-zA-Z0-9]*/,
+    _camelCaseIdentifier: ($) => /[a-z_]+[a-zA-Z0-9_]*/,
+    _pascalCaseIdentifier: ($) => /[A-Z_]+[a-zA-Z0-9_]*/,

Verified locally by regenerating the parser and re-parsing extends/implements/new X(...)/leading-underscore cases — all resolve with no ERROR nodes. A downstream tree-sitter consumer (a Haxe-aware code-graph extractor) now correctly resolves inherits/instantiation edges for identifiers using this naming convention, where previously they silently collapsed to whatever text preceded the first underscore. Not including the regenerated src/ output in this PR itself, since the project's convention (per past merged PRs) is grammar-source-only and letting the maintainer regenerate.