* feat(themes): public plugin theme tokens for chat text and selection
Themes can now set chat bold text, muted text, inline code, code blocks and
the chat selection. Omitted tokens paint exactly what chat painted before,
except the selection, which now keeps selected text 4.5:1 by default.
* docs(themes): note the bubble and code-block checks for chat text tokens
* fix(themes): never lower the chat selection under its visible floor