feat(lsp): npm specifier completions

[nayeemrmn]

Closes #15811.

Snipping tool screen recorder isn't working so here's some screenshots:

[nayeemrmn]

What kind of test would we allow for this?

[ry]

Nice work. I think there are some existing completion tests in the LSP code that could be copied for this?

[nayeemrmn]

This one depends on the npm registry site... I think I'll swap out the endpoint URLs in a cfg(test) to a localhost with fake responses.

[bartlomieju]

This one depends on the npm registry site... I think I'll swap out the endpoint URLs in a cfg(test) to a localhost with fake responses.

That's a great idea.

[nayeemrmn]

Okay we already have a CliNpmRegistryApi abstraction whose origin can be overrided with an env var which npm tests use.

[nayeemrmn]

I wanted to reuse the loading stuff in CliNpmRegistryApi but when I tried to move the search API request over there was a lot of concurrency and cache machinery that looked overkill for the LSP. That means there are now two spots where package info is fetched.

[dsherret]

Thanks! This will be a big improvement.

[dsherret]

Almost there. Given the changes made that expose the high level construction code to the unit tests, I think we should make a seam for the api provided to get_npm_completions so we don't need to expose create_npm_api_and_cache.

[dsherret]

Sorry for my delay on this one! I thought it was good and we were just waiting for the merge window to open, but then I just looked at it a bit more and I think it will only use the cached package information. Additionally, we could decouple get_npm_completions from deno_npm's NpmRegistryApi trait.

[dsherret]

Looks great. Nice work and thanks @nayeemrmn!