feat: Added SpriteFont class

[st-pasha]

Description

This PR adds the SpriteFont class, which encapsulates reusable information about a sprite font. Text renderers / text formatters can be created based on this SpriteFont.

In addition:

• GlyphInfo removed (was internal), and GlyphData deprecated -- instead, there is now a consolidated Glyph class;
• Sprite font now supports extended Unicode characters and ligatures (when a combination of letters is represented as a single glyph)
  • This feature can also be used to define simple name substitutions. For example, you can declare that "[gp]" or "&gp;" would represent a gold coin icon;
• Sprite font supports variable-width fonts;
• Sprite font supports glyphs where the source rect is different from the logical rect;
• Sprite font renderer can now control the color of the text rendered.

Checklist

• The title of my PR starts with a Conventional Commit prefix (fix:, feat:, docs: etc).
• I have followed the Contributor Guide when preparing my PR.
• I have updated/added tests for ALL new/updated/fixed functionality.
• I have updated/added relevant documentation in docs and added dartdoc comments with ///.
• I have updated/added relevant examples in examples or docs.

Breaking Change?

• [-] Yes, this PR is a breaking change.
• No, this PR is not a breaking change.

Related Issues

Closes #1935

[spydon]

Lgtm! Some minor comments