fix: Improve rendering of text in response to theme and CSS changes

[gonfunko]

The basics

• I validated my changes

The details

Resolves

• Fixes Renderer CSS is not refreshed when the theme's fontStyle changes (regression from #9611) #10562
• Fixes Text width is calculated incorrectly with custom external CSS #10093
• Fixes dynamic font sizing doesn't work with new render management #7782
• Fixes Warning bubbles do not resize on text resize #3706
• Fixes Comment text should change size with block text #3707
• Fixes Labels With Different Font Size are Not Rendered Correctly #2694

Proposed Changes

This PR fixes a whole bunch of tangentially related bugs related to text and element sizing depending on themes and CSS. In particular:

• When a Blockly theme is set that has the same name as the current theme, its CSS will be injected, overwriting the old values for that theme.
• When a theme is set that specifies a font style (e.g. size), the bounds of all fields on the current set of blocks will be invalidated, causing the change to font style to be immediately reflected. This fixes the Rendering > font Size slider on the advanced playground, which has been broken since v10.
• When a theme's text size changes, warning bubbles and comments have their text resized accordingly.
• If text on certain blocks is targeted with CSS to apply specific font attributes, those will be respected, and the blocks will be sized to fit their text.
• letter-spacing and word-spacing in a user stylesheet (or the stylesheet of a site that uses Blockly) are respected when rendering blocks.

A demo of all of the above:

scaling.mov

Reason for Changes

We should support letter-spacing and word-spacing for accessibility, themes should work as documented, and blocks and other workspace elements should respond to text size changes and resize to fit their contents.