Add Nix language support

[TyceHerrman]

Supersedes #219 and #330.

Summary

• Adds Nix .nix detection, display name, Language entry, extractor registration, and vendored tree-sitter-nix WASM with no new package dependency.
• Uses feat: add Nix language support #330 as the functional base because it carried the broader extractor and resolver behavior; credits feat: add Nix language support #330 for that implementation and feat: Nix support #219 for the README/default include intent.
• Extracts Nix variables vs functions, simple/destructured/curried params, inherit bindings, static import refs, function call refs, and exported vs private attrset scope.
• Resolves only static relative Nix path imports to indexed file nodes, including ./dir to dir/default.nix. Angle-bracket channels, attribute expressions, and variable/dynamic imports do not create project file edges.
• Keeps existing import resolution behavior by scoping Nix file-path imports beside the existing C/C++ and PHP file-import branches.

Notes

• Current main no longer has DEFAULT_CONFIG. Nix is included through EXTENSION_MAP/isSourceFile, which is the current indexing source of truth.

Test plan

• npm run build
• npm test -- tests/extraction.test.ts tests/resolution.test.ts
• npm test was attempted locally, but the full suite entered a broad unrelated 5s timeout cascade across many heavy suites under local resource pressure, so it was stopped.