[google_fonts] Decouple GoogleFontsLite for full tree-shaking and add full feature parity

[Piinks]

In PR #11433 #11433, google_fonts_lite.dart was introduced as an alternative entry point containing only dynamic font resolution methods and the family descriptor map, allowing compilers to tree-shake the static generated font methods.

#11433 was a huge PR, and a breaking one, so we did not publish it yet. Currently this package is set to publish_to: none.

For these reasons I decided to follow up on a few more bits of polish here instead of belaboring the mega PR and the contributor's effort.

After this change, I will migrate this package to material_ui and then we will publish one major release with all of these changes to reduce churn on the ecosystem.

This PR ensures google_fonts_lite.dart is fully decoupled from the main entry point by sharing an internal configuration singleton, ensuring complete compiler tree-shaking of unused generated font code, while adding missing features to google fonts lite for full feature parity with the non-lite version.

Pre-Review Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
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• I signed the CLA.
• The title of the PR starts with the name of the package surrounded by square brackets, e.g. [shared_preferences]
• I linked to at least one issue that this PR fixes in the description above.
• I followed the version and CHANGELOG instructions, using semantic versioning and the repository CHANGELOG style, or I have commented below to indicate which documented exception this PR falls under¹.
• I updated/added any relevant documentation (doc comments with ///).
• I added new tests to check the change I am making, or I have commented below to indicate which test exemption this PR falls under¹.
• All existing and new tests are passing.

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Footnotes

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Code Review

This pull request decouples the internal base library from the main entry point to ensure complete tree-shakability, and adds config, pendingFonts, and getTextTheme to GoogleFontsLite along with documentation and tests. The review feedback suggests throwing an ArgumentError instead of a generic Exception when a font family is not found in getTextTheme, and updating the corresponding test to match.