deprecate heading cells in UI

- removed from Kernel Menu
- Main toolbar warns via shouty dialog
- keyboard shortcuts still work to make headings in markdown cells, with no warning
Merge pull request #6846 from minrk/deprecate-heading-ui

deprecate heading cells in UI
distinguish capture_output from buffer_output

allow capturing output without silencing it,
which is useful for slimerjs,
which requires looking at output to determine failure.
Merge pull request #6860 from minrk/slimerjs

distinguish capture_output from buffer_output
Update components to codemirror 4.7
Merge pull request #6865 from takluyver/cm4.7

Update components to codemirror 4.7
allow multiple forms of latex math in test_markdown

different versions of pandoc use different escapes
Merge pull request #6863 from minrk/markdown-tex-math

allow multiple forms of latex math in test_markdown
upgrading >v4 header cells to unicode

adding test
more human-readable unicode chars in test
Merge pull request #6868 from bollwyvl/unicode-upgrade-headers

upgrading >v4 header cells to unicode