fix(extract): emit inherits edge for JavaScript class extends

[Synvoya]

Bug

class Derived extends Base {} in a plain .js file produces no inherits edge, while the identical code in a .ts file works.

Root cause

The two tree-sitter grammars shape class heritage differently:

• TypeScript: class_heritage → extends_clause → identifier (base wrapped in a clause node)
• JavaScript: class_heritage → extends, identifier (base held directly, no extends_clause wrapper)

_ts_walk_class_members (graphify/extractors/resolution.py) only iterated class_heritage children looking for extends_clause / implements_clause. For .js classes neither exists, so the heritage entry was silently dropped.

Minimal repro

// a.js
class Animal {}
class Dog extends Animal {}

extract(["a.js"]) yields no Dog --inherits--> Animal edge. The equivalent a.ts already emits it.

Fix

When a class_heritage node has no extends_clause/implements_clause child, treat the heritage node itself as the extends clause and emit an inherits edge — mirroring the existing extends_type_clause branch used for interface heritage (#1095). TypeScript files are unaffected (they still take the clause branch, guarded by a saw_clause flag, so there is no double emission).

Testing

• Added test_js_class_extends_same_file to tests/test_ts_inheritance.py, mirroring the existing test_class_extends_same_file (.ts) test but with a .js file. It fails before the change and passes after.
• tests/test_ts_inheritance.py: 8 passed.
• tests/test_languages.py: 314 passed, 13 skipped (unchanged).
• JS/TS import-resolution, receiver, and decorator suites: 70 passed.