extensions: Add support for snippets provided by extensions (#14020)

For now extensions can only register global snippets, but there'll be
follow-up work to support scope attribute in snippets.json.

Release Notes:

- Extensions can now provide snippets by including `snippets.json` file
next to the extension manifest.

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Co-authored-by: Marshall <marshall@zed.dev>
Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>