chore(react-icons): improve font/svg generator performance and add svg-sprites opt-in generators

[Hotell]

Summary

This PR introduces:

• existing svg/font generators refactoring for better perf and simpler code
• SVG sprite generation support for @fluentui/react-icons as a preview feature, gated behind an opt-in --sprites CLI flag.

NOTE: Sprites are not included in stable releases.

Generators Refactoring & Fixes

• convert.js — processPerIcon() now accepts optional spriteDest parameter; sprite metadata is only computed when sprites are enabled.
• convert.utils.js — generatePerIconFiles() refactored to handle both per-icon atoms and sprite generation in a single pass.
• per-icon.writer.js — Refactored grouping logic; shared between SVG atoms and sprite writer.
• convert-font.js / convert-font.utils.js — Refactored generatePerIconFiles signature to accept { resizable, sized } object instead of flat array.
• convert-font.utils.test.js — Fixed test calls to match the refactored generatePerIconFiles signature (was passing flat array, now passes { resizable: [], sized: entries }).
• merge-metadata.js — Added log start/end blocks.

SVG Sprites

SVG Sprite Generation Pipeline (new)

• sprite.writer.js — New module that generates SVG sprite pairs (.svg + .tsx) from the same icon data used for per-icon atoms. Each sprite file contains <symbol> elements for
  all icon variants, and a companion TSX module that uses createFluentIcon with a sprite URL reference.
• src/utils/createFluentIcon.svg-sprite.tsx — New createFluentIcon factory for sprite-based icons that renders <use href="..."> references instead of inline SVG paths.
• src/assets.d.ts — merged TypeScript declaration for .svg and font static asset imports.

Opt-in --sprites Flag

• convert.js — Added --sprites boolean CLI flag to parseArgs(). When absent (default), --spriteDest is ignored and all sprite generation, directory creation, and metadata
  writing are skipped. When --sprites is passed, --spriteDest is required and sprites are generated as before.
• build.js — Sprite SVG asset copying (src/atoms/svg-sprite/*.svg → lib/ and lib-cjs/) is guarded behind an existsSync() check. When sprites are present,
  addSpriteExportMap() dynamically injects the ./svg-sprite/* export map entry into package.json.
• package.json — Removed static ./svg-sprite/* export map entry. It is now generated at build time only when sprites are enabled.

Documentation

• docs/preview-features.md — Documentation for the SVG sprite preview feature.
• README.md — Added reference to preview features docs.

How to Enable Sprites

Add --sprites to the convert:svg script and restore the ./svg-sprite/* export map in package.json:

# In convert:svg script, add --sprites flag:
node convert.js --source=./intermediate --dest=./src --perIconDest=./src/atoms/svg --spriteDest=./src/atoms/svg-sprite --sprites --rtl=./intermediate/rtl.json 
--metadata=./tmp/metadata-svg.json

When sprites are stabilized, the --sprites flag can be added permanently and the export map will be auto-generated during build.

[Hotell]

this is necessary as our infra uses latest node / TS 5+ which is hoisted to workspace root

[ValentinaKozlova]

If color variants will be deprecated, do we need tests for them?

[Hotell]

we do,

deprecated != removed.

to maintain API contract we can remove it only with major bump -> 3.0.0