Move extension to TypefaceFontProvider instead of SkFontMgr

[srujzs]

This extension should belong on TypefaceFontProvider instead of the parent class. The only usage is in lib/web_ui/lib/src/engine/canvaskit/fonts.dart, and both usages are on a variable typed as TypefaceFontProvider. In the future, multiple extensions on the same @staticInterop type will be an error (https://github.com/dart-lang/sdk/issues/49350), so this must be migrated.

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• [X] I read the [Contributor Guide] and followed the process outlined there for submitting PRs.
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[srujzs]

cc @yjbanov PTAL

[flutter-dashboard[bot]]

It looks like this pull request may not have tests. Please make sure to add tests before merging. If you need an exemption to this rule, contact Hixie on the #hackers channel in Chat (don't just cc him here, he won't see it! He's on Discord!).

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[Hixie]

test-exempt: this effectively removes (broken) API surface, so there's nothing to test really.

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Validations Fail.