fix: resolve package subpath exports

[pkudinov]

Summary

• resolve workspace package subpath imports through package.json exports
• support exact subpath exports such as ./browser
• support wildcard subpath exports such as ./server/queries/*
• prefer source-oriented conditions before runtime/type-only targets

Fixes #1308.

Verification

• uv run pytest tests/test_js_import_resolution.py

[safishamsi]

Thanks @pkudinov — the extract.py change is correct and well-scoped: it handles exact subpath exports, conditional exports (source/import/require/types, source-first), and wildcard subpaths, and it's additive (export candidates are tried first, the package_dir / subpath fallback is preserved), so no regression risk to existing resolution.

The only blocker is a rebase: #1352 (npm/yarn workspaces) landed first, and both PRs added tests at the same anchor in tests/test_js_import_resolution.py, so this is now CONFLICTING. The conflict is only in the test file — extract.py auto-merges cleanly. Please rebase onto current v8 and keep both sets of tests (trivial concatenation); then CI can run and I'll merge.

Optional follow-up nit: also handle array-form export targets ("./x": ["./a.ts", "./b.ts"]), which currently return no candidates.

[pkudinov]

Rebased this PR onto current v8 and resolved the test-file conflict by keeping both the npm/yarn workspace coverage and the pnpm package-subpath export coverage.

I also included the optional array-form export target handling for entries like "./x": ["./a.ts", "./b.ts"].

Verification: uv run pytest -q passed with 2098 passed, 28 skipped.