Fix FileSystemProvider mtime caching

[isc-bsaviano]

This PR fixes FileSystemProvider mtime caching. Previously, the cache would get constantly overwritten so calls to GET /doc wouldn't include the IF_NONE_MATCH header, forcing the server to send the full contents of the file back every time. This fix corrects this problem so the proper mtime is sent with GET /doc requests.

[gjsjohnmurray]

Based on code-reading this looks OK to me. I haven't tested the dev VSIX though.

[isc-bsaviano]

@gjsjohnmurray @daimor There's something wrong with the CI. Compile fails because it can't find the vscode module. I'm not going to merge this until we get a CI run for this PR that succeeds

[daimor]

There is something wrong with downloading vscode.proposed.d.ts

[daimor]

microsoft/vscode#136964

[gjsjohnmurray]

microsoft/vscode#136964

Correct. The CI needs to be revised to handle the relocation of vscode.d.ts and the breaking up of vscode.proposed.d.ts.

But I think the reason this is suddenly biting us is that the CI downloads these from main (actually, still using the old name 'master'). It should probably be using npx instead.

[daimor]

I'll take care of this change, and create a PR for it

[isc-bsaviano]

@gjsjohnmurray We have a dev vsix for this now if you want to test it before I merge

[gjsjohnmurray]

A quick test of the dev vsix didn't find anything broken, so I'm happy for my approval to stand.