feat: add character style support for text casing

[Omikhleia]

Less obvious than it seems, due to some command expecting text/tree content...

Needs

• [x] Add case support in character styles
• [x] Refactor style cascading support and explain why the "naive" approach fails... --> See comment below
• [x] Documentation

[Omikhleia]

Refactor style cascading support and explain why the "naive" approach fails...

See https://github.com/Omikhleia/markdown.sile/pull/80 for context in a simpler case. But here, this is getting messy as hell, for a use case which was just to support text case in styles initially...

Raising questions to SILE for possible advice ^^ https://github.com/sile-typesetter/sile/discussions/1834

[Omikhleia]

Moving forward: needed in production...