ci: Build .vsix and add to release assets

[gadenbuie]

Adds a new job to automate building the vsix and adding to the release assets when a release is created.

Uses https://github.com/Shopify/upload-to-release

[jcheng5]

1. Is the existence of the archives on VS Code Marketplace and Open-VSX not enough for you?
2. Does it bother you at all that this is not the same binary as is actually published on those marketplaces? (They won't have the same hashes, even if the contents are the same)

[jcheng5]

I don't think we need this, but if you still want it I don't object.

[gadenbuie]

• Is the existence of the archives on VS Code Marketplace and Open-VSX not enough for you?

I hadn't realized they existed. Idk, there's something appealing about keeping the history closer to the source. It occurred to me to look through our releases for the history but not the marketplace.

• Does it bother you at all that this is not the same binary as is actually published on those marketplaces? (They won't have the same hashes, even if the contents are the same)

Not at all, it was more about keeping an artifact that could be used to manually install the extension at that point in time.

I don't think we need this, but if you still want it I don't object.

I have similar ambivalence, so I'll close this for now. If I feel differently in the future we can pull this back out.