isSingleGist is too broad and incorrect

[fregante]

1. These two (old) URLs are not detected as isSingleGist:

https://gist.github.com/fregante/2205329b71218fa2c1d3
https://gist.github.com/fregante/2205329b71218fa2c1d3/d1ebf7d9cfaba4d4596d2ea9174e202479a5f9ad # this is a specific gist revision

2. This is also detected as isGist, but is actually a /raw/ file, not a gist at all:

https://gist.github.com/fregante/2205329b71218fa2c1d3/raw/d1ebf7d9cfaba4d4596d2ea9174e202479a5f9ad/iframe.js

3. This is correctly detected as isGist, but maybe we should have a more specific isSingleGistCode so that only the code tab is detected.

https://gist.github.com/fregante/2205329b71218fa2c1d3/revisions

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These changes should probably be based on https://github.com/sindresorhus/refined-github/blob/1a9c4f9b/source/features/embed-gist-inline.tsx#L9-L23 and will replace that inline function (which intends to be isSingleGistCode, but is more like a more specific isSingleGist)

Go to demo: https://fregante.github.io/github-url-detection/

[fregante]

3. This is also detected as isGist, but is actually a /raw/ file, not a gist at all:

This is actually fine. isGist means it's on the Gist website. I don’t think we have isRaw detections since extensions don't need to work there