Add H4-H6, strikethrough formatting & buttons

[riasvdv]

Preemptive PR for panphora/markdown-actions#1 and panphora/markdown-actions#2

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Summary by cubic

Adds H4–H6 header support and strikethrough formatting across the editor. Updates the parser, toolbar, icons, styles, themes, docs, and TypeScript types; the default toolbar now includes Strikethrough.

• New Features
  • Parser: supports #–###### headers with tests for H4–H6.
  • Toolbar/UI: adds strikethrough, h4, h5, h6 buttons; default includes strikethrough; new icons.
  • API/Types: exposes markdownActions.toggleStrikethrough and toggleH4–toggleH6; adds matching toolbarButtons; updates overtype.d.ts and generated types.
  • Styles/Themes/Docs: colors and preview sizes for H4–H6; updates solar and cave themes; README documents new actions and supported headers.

^{Written for commit e16928f. Summary will update on new commits.}