JS/TS resolution: honor baseUrl declared in an extended tsconfig, and add a JSONC config test

[safishamsi]

Follow-up to #2153 / #2176 (baseUrl/paths resolution). Two gaps from review:

1. extends-inherited baseUrl is not honored. _load_tsconfig_base_url parses only the leaf config, so a tsconfig.json with "extends": "./tsconfig.base.json" where the base declares baseUrl resolves to None (common monorepo/NestJS shape). Mirror the extends-chain handling that _read_tsconfig_aliases already has (child declaration wins; resolve each baseUrl relative to the config that declares it; skip @-scoped npm configs; circular-guard).
2. Also apply an isinstance(compilerOptions, dict) guard so a "compilerOptions": null config does not raise AttributeError (pre-existing in the alias loader too).
3. Add a JSONC test: a jsconfig with // comments, /* */, and trailing commas with a baseUrl resolves (the code already strips JSONC via the _read_tsconfig_aliases() silently fails on tsconfig.json with comments — every JS/TS project hits this #700 helper, but there is no test).

[siddharthgoyal00]

can i work on this issue!