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

Each test now owns a minimal fixture and asserts a single behavior, replacing
the previous setup where 17 cases shared a single large fixture inside one
test.each block. Adds:

- setupFixture in e2e-test/test-util/fixture.ts, with getLoc / getRange for
  string-based position lookup; 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