refactor(ts-plugin): split go-to-definition e2e tests into per-behavior cases (#378)

Each test now owns a minimal fixture and asserts a single behavior, organized
by the .d.ts/mapping pattern under test. The previous structure shared one
large fixture (5 files, 8+ tokens) across 17 cases inside a single test.each
block, which made it brittle to add new behaviors.

The new structure has 14 tests grouped into 4 describe blocks:
- jumps to the top of a CSS module file (5 tests)
- jumps to a local token declaration (3 tests)
- transitively resolves a re-export to its source declaration (3 tests)
- jumps from a CSS-side @value ... from binding to its source declaration (3 tests)

Adds:
- setupFixture / getLoc / getRange in e2e-test/test-util/fixture.ts. The API
  mirrors createIFF and accepts files directly (including tsconfig.json), so
  each test controls its own config placement.
- buildStylesImport / buildTSConfigJSON in packages/ts-plugin/test/builder.ts
  for building default / namespace import statements and tsconfig.json fixture
  content. cmkOptions.enabled defaults to true so callers only specify the
  options that matter for the test.

Author: mizdra