fix uri fragment slugification #94508

[iliazeus]

Previously, all links to local files had their fragment parts normalized (e.g. foo.md#Bar baz became foo.md#bar-baz). This allowed links to specific sections of other markdown files (file#Section header), but broke many other links.

This PR limits the normalization to paths without a file extension (as per suggestion).

This may, however, have an unfortunate side effect of breaking some markdown links that do have an extension (e.g. foo.md#Bar baz won't work, but foo#Bar baz will).

This PR fixes #94508

[iliazeus]

Added an additional check for known markdown file extensions. Now links are slugified if they have a markdown file extensions or do not have an extension at all.

[mjbvz]

Thanks! This change will be in the next VS code insiders build and is scheduled to go out with VS Code 1.45