Support extentions providing code snippets via registered text document content providers (fix #248407)

[mbehzad]

Hi, this PR implements what was suggested in this issue , which is adding support for extensions defining the path to the snippet file as a custom scheme. e.g.

"contributes": {
    "snippets": [
      {
        "language": "html",
        "path": "my-scheme:/snippets.json"
      }
    ],

Regarding testing, I have created a simple extension based on contentprovider-sample, which you can find it here: https://github.com/mbehzad/snippet-extension-test It can be run via its launch.json or installing its vsix package that is also in the repo here in the vscode from this PR.

Cheers, Mehran

[mbehzad]

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