Add opt-in sourceMap option to emit .css.map files and sourceMappingURL comments alongside compiled CSS

[cmalonzo]

Summary

Fixes sp-dev-docs#10831 — SPFx 1.23 dev builds lost CSS/SCSS source-map support because @rushstack/heft-sass-plugin never passed sourceMap: true to sass-embedded, so no .css.map files were produced and DevTools "Sources" pointed at intermediate compiled files instead of the original .scss.

Changes

src/SassProcessor.ts

• Added sourceMap?: boolean to ISassProcessorOptions
• When sourceMap: true, passes sourceMap: true and sourceMapIncludeSources: true to compileStringAsync so result.sourceMap is populated
• For each CSS output folder: writes a <file>.css.map alongside the .css, with sources[] rewritten from internal heft: URLs to paths relative to the map file's directory (what source-map-loader expects); appends /*# sourceMappingURL=<basename>.css.map */ to the .css
• Default is false — no behavior change for existing consumers

src/SassPlugin.ts

• Added sourceMap?: boolean to ISassConfigurationJson and threads it through to ISassProcessorOptions

src/schemas/heft-sass-plugin.schema.json

• Added "sourceMap" boolean property

src/test/SassProcessor.test.ts

• Added three tests under describe('sourceMap option'):
  • sourceMap: true emits a .css.map and a sourceMappingURL comment; map is valid JSON v3 with non-empty mappings and sources[0] resolving to the input .scss
  • Default (sourceMap unset) produces no .css.map and no sourceMappingURL comment, confirming byte-identical output to pre-change behavior
  • sourceMap: true + doNotTrimOriginalFileExtension: true uses the correct foo.scss.css.map filename in both the written file and the sourceMappingURL comment

Downstream contract

The plugin's only obligation to consumers is:

• A parseable v3 source map at <file>.css.map
• A /*# sourceMappingURL=<basename>.css.map */ comment in the emitted .css
• sources[] entries that resolve (relative to the .css.map) to the original .scss on disk

[cmalonzo]

Blocker for SharePoint/sp-dev-docs#10831