Search and replace with substring in markdown and raw cells

[JasonWeill]

Fixes #16292.

Adapts the codemirror searchprovider changes from #15881 to also apply to the MarkdownCellSearchProvider. Updates unit tests to test all pairs of code, markdown, and raw cells.

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[srdas]

@JasonWeill - I installed, built, and tested the new version from your branch search-replace-substring-markdown and the find and replace is now working as expected across all types of cells (code, markdown, raw).