Don't focus trap tab by default

[riasvdv]

Fixes #113

• Collapsed Tab/Shift+Tab now fall through to native focus behavior, while selected text still indents/outdents.
• Added public indentSelection() / outdentSelection() line transforms, using two spaces and setRangeText(..., 'preserve').
• Added Ctrl/Cmd+] and Ctrl/Cmd+[ to shortcuts.
• Added preview inert / aria-hidden syncing

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Summary by cubic

Stop trapping focus on Tab: collapsed Tab/Shift+Tab now move focus while selections still indent/outdent (Fixes #113). Also adds indent/outdent shortcuts and improves preview accessibility.

• New Features

  • Public APIs: indentSelection() and outdentSelection().
  • Shortcuts: Ctrl/Cmd + ] to indent, Ctrl/Cmd + [ to outdent.

• Bug Fixes

  • Tab no longer traps focus; with a selection it indents/outdents, otherwise it follows native focus.
  • Preview stays out of the tab order with inert/aria-hidden until Preview mode.
  • Prevent duplicate global listener initialization.

^{Written for commit a554161. Summary will update on new commits.}