Add H4, H5 and H6 operations

[riasvdv]

insertHeader has support for H1 through H6, but markdown-actions only had operations for H1-H3, this adds H4 through H6 operations and detection

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Summary by cubic

Adds H4–H6 header operations and detection to markdown-actions, so users can toggle and detect headers up to H6. Aligns the UI and detection with existing insertHeader support.

• New Features
  • Added toggleH4, toggleH5, toggleH6 APIs.
  • Extended getActiveFormats to detect header-4, header-5, header-6.
  • Exported new toggles and added H4–H6 buttons in test-browser.html.

^{Written for commit d2cfd15. Summary will update on new commits.}