Runtime modifiers

Finally ready! Some highlights:
- Text objects own TextModifierGroups
- A TextModifierGroup defines a set of operations (Modifiers) to perform on ranges of the whole Text object.
- The ranges are defined by TextModifierRanges which are owned by TextModiferGroups. Each TextModifierGroup can have multiple TextModifierRanges, meaning that the set of Modifiers will perform on the final selection defined by TextModifierRanges.
- TextModifierRanges select portions of the text in a Text object based on rules that define the strength of the selection. The strength of multiple TextModifierRanges can be blended together meaning selection strength can be above 1 and below 0 (optionally clamped).
- There are three fundamental modifications:
  1. Transform where glyphs are individually transformed by a matrix
  2. Paint where glyphs are drawn with different paint (currently only opacity is supported, but later Fill and Stroke will be too).
  3. Shape where a selection can have shaping properties (currently variable axes later also font size) changed. This is the most complex one as it requires [two pass shaping](https://github.com/rive-app/rive/blob/ef2d0d3ca32b6c61595d94587fb22f6456478896/packages/runtime/src/text/text.cpp#L578-L598).

Diffs=
8086bdbf9 Runtime modifiers (#5512)