[heft-sass-plugin] Emit declaration source maps for generated typings

[mikedelgaudio]

Summary

Fixes #5908

Sass typings are written into a folder that rootDirs merges into the source tree, so the language
service only ever sees the generated .d.ts. "Go to definition" on a CSS module class therefore
stops at the generated declaration instead of opening the rule that declares it.

This adds an opt-in generateDeclarationMaps option that emits a .d.ts.map beside each generated
typings file. It is off by default, and when disabled the output is unchanged.

This is the Sass counterpart to #5906 / #5907, which cover localization typings. It is stacked on
#5907 — the shared declaration-map primitives live in @rushstack/typings-generator, so this
branch contains that commit and should merge after it.

Details

The interesting part is obtaining accurate positions. Sass is compiled before PostCSS runs, so
PostCSS positions refer to the compiled CSS rather than the stylesheet. The chain is:

1. Request a source map from Sass when the option is enabled.
2. Record where each class selector appears in the compiled CSS, using a PostCSS plugin registered
   before postcss-modules (which rewrites class names).
3. Translate that position back through the Sass source map to the original stylesheet.
4. createDTS reports the line it emits each declaration on, so the generated side needs no parsing
   of its own output.

Because the lookup happens in compiled-CSS order, this gets two cases right that a text search over
the stylesheet cannot:

• A class declared in an @imported partial resolves into that partial. It does not appear in
  the importing file at all.
• A class restated inside a @media or theme block still resolves to its primary rule, with no
  indentation heuristics.

Shared vs. Sass-specific code

Per the two questions raised in #5908, and to avoid a third copy of this logic appearing downstream:

Package	Change
@rushstack/typings-generator	serializeDeclarationMap now accepts multiple sources, and IDeclarationMapping gains an optional sourceIndex — needed because Sass declarations can originate from several files. Adds decodeMappings and originalPositionFor alongside the existing encoder.
@rushstack/heft-sass-plugin	New SassDeclarationMaps.ts with the PostCSS recorder and the resolve-through-Sass-map step, exported from the package index so other Sass typings generators can reuse it. Adds a dependency on typings-generator.

Notes for reviewers:

• Backwards compatible. serializeDeclarationMap's sources parameter widens from string to
  string | readonly string[], and sourceIndex defaults to 0, so existing callers are
  unaffected. The heft-localization-typings-plugin path is unchanged.
• sourceMapIncludeSources is not forced on. Enabling declaration maps requests a Sass source
  map, but only embeds sources when sourceMap: true was already set, to avoid inflating output.
• Happy to split the typings-generator half into its own PR if you would prefer to review the
  shared primitives separately from the Sass plugin.

How it was tested

Validated on Linux / Node 22.

• rush build and rush test clean for heft-sass-plugin, typings-generator, and
  localization-utilities.
• heft-sass-plugin: 60/60 tests pass, including 3 new cases that decode the emitted map and
  assert the resolved line:
  • each declaration resolves to the rule that declares it;
  • a class declared in a partial resolves into _partial-with-class.scss, not the importing file;
  • a class restated inside @media resolves to its primary rule, not the restatement;
  • no .d.ts.map is emitted when the option is disabled.
• typings-generator: the 26 existing tests from [typings-generator] Emit declaration source maps for generated typings #5907 still pass unmodified, confirming the
  multi-source change did not regress the single-source path.
• Separately verified the same approach against a live tsserver in a large internal monorepo:
  go-to-definition on a CSS module class resolved to the .scss rule rather than the generated
  .d.ts.

Impacted documentation

• heft-sass-plugin gains a generateDeclarationMaps option; the plugin's configuration docs would
  need a new entry. The JSON schema is updated in this PR.
• rush change files are included, and common/reviews/api/typings-generator.api.md is updated.