[google_fonts] Add google_fonts_lite file to allow tree-shaking of the other huge files

[TheCarpetMerchant]

Adds a google_fonts_lite.dart generated file, which contains a map of fontFamily to fonts variants and a getFont function that simply forwards the call to the internal googleFontsTextStyle function which does the downloading and loading of the font.

List which issues are fixed by this PR. You must list at least one issue.
flutter/flutter#184337

I have not done documentation, testing etc because this PR is for discussion purposes. I'd like approval of the concept before doing the paperwork.

The idea is to provide a lite version of the GoogleFonts class. This lite version does not depend on any of the other generated code, which is absolutely huge and takes up a massive amount of space (8MB on android for v8.0.0). By using this lite version only, the compiler is able to tree-shake all that additionnal code while you retain the ability to invoke any font from the package via getFont.

For me this brings the package's footprint from 8MB/target platform to less than a 1MB/target when building a release apk.

[flutter-dashboard]

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[gemini-code-assist]

Code Review

This pull request introduces a 'lite' version of the Google Fonts generator to support tree-shaking by creating a GoogleFontsLite class. Key changes include updating the generator to process a new template, making the GoogleFontsFile constructor const, and adding the google_fonts_lite.tmpl file. Review feedback recommends fixing a typo in the template comments, declaring the GoogleFontsLite class as abstract with a static const map, using display names for font keys, and implementing safer error handling in the getFont method to replace the current null-assertion operator.

[gemini-code-assist]

Typo: "WHat" should be "What".

// What is called is googleFontsTextStyle, which attempts to load a font if not already loaded.

[gemini-code-assist]

Consider making GoogleFontsLite an abstract class to prevent instantiation, as it only contains static members. Additionally, the fontsMap should be declared as static const for better efficiency. Using fontFamilyDisplay (the actual font name with spaces) as the map key is also recommended for better compatibility with font fallback and consistency with the main GoogleFonts API.

abstract class GoogleFontsLite {
  static const Map<String, Map<GoogleFontsVariant, GoogleFontsFile>> fontsMap = {
    {{#fontEntries}}
      '{{fontFamilyDisplay}}': {
      {{#fontUrls}}
      const GoogleFontsVariant(fontWeight: FontWeight.w{{variantWeight}}, fontStyle: FontStyle.{{variantStyle}},) : const GoogleFontsFile('{{hash}}', {{length}},),
      {{/fontUrls}}
      },
    {{/fontEntries}}
  };

[gemini-code-assist]

The getFont method uses the null-assertion operator !, which will cause a runtime crash if an unknown font family is requested. It is safer to check for the existence of the font and throw a more descriptive error.

  static TextStyle getFont(String fontFamily) {
    final fonts = fontsMap[fontFamily];
    if (fonts == null) {
      throw ArgumentError('No font family with name $fontFamily was found.');
    }
    return googleFontsTextStyle(fontFamily: fontFamily, fonts: fonts);
  }