Preserve file names for language detection

[iliazeus]

VSCode is able to detect and apply language modes automatically, looking at (among other things) file extensions. This PR makes it so the ContentProvider URIs preserve extensions of their source files. That enables VSCode to set language modes by default.

[iliazeus]

Resolves #38

[pawk3k]

I can add one more why we need this ?
In case of someone using https://semanticdiff.com/, file extension is needed to better understand how to calculate diff for better visibility @ryu1kn , could you please take a look into it 🙏

[iliazeus]

@ryu1kn can you please review this PR? This is the single feature I miss from this extension.

[ryu1kn]

Apologies for not attending the PR for years @iliazeus . Also thanks for the additional context @pawk3k .

The concern i had here is that since partial diff can grab arbitrary portion of code to compare, the part extracted may not be well-formed, and get red squiggles, which can be noisy.

I've just tried your branch. It looks like this. Since it's just red squiggles, probably it's not too bad, and users would know what they're doing. I'm happy to introduce this as a toggled feature. Something like partialDiff.enableFileTypeRecognition (boolean, default false)?