refactor: establish CSSFontLoading folder feature

[jderochervlk]

Tracking issue: #342

Stack context

Part 8 of the Option 5 folder-feature stack.

• Previous: refactor: establish Crypto folder feature #317
• Next: another dependency-clean domain folder

Review this PR as the incremental diff against #317.

Summary

• rename src/CSSFontLoading to the target src/css-font-loading folder
• move BaseCSSFontLoading out of core and into its owning complete folder
• change the folder to the unqualified internal CSSFontLoading feature
• add the supported WebAPI.CSSFontLoading feature with its WebAPI.Event dependency
• preserve the flat WebAPI.FontFace and WebAPI.FontFaceSet public modules

Temporary state

• this PR removes the temporary BaseCSSFontLoading placement documented in refactor: establish DOM core folder #311
• the internal BaseCSSFontLoading and CssFontLoadingTypes aggregation remains inside the correct folder; any final type colocation is deferred to cleanup
• no file exclusions or module slices are introduced

Review focus

• the WebAPI.CSSFontLoading -> WebAPI.Event + CSSFontLoading implication
• complete folder membership, including the formerly foundational helper
• unchanged public module paths and signatures

Verification

• node_modules/.bin/rescript clean
• node_modules/.bin/rescript build --prod --features DOM,Event,CSSFontLoading
• npm run build
• npm test
• npm run format:check
• git diff --check

[jderochervlk]

@codex

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. Can't wait for the next one!

Reviewed commit: d6df25203b

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