(perf) cache fileExists resolutions

[dummdidumm]

This makes getCompletions perform many times faster, the bigger the project the more it benefits from this cache.

I noticed that there's a lot of time spent in fileExists during analyzing whether or not #1192 does make any difference. I then also found that TypeScript does something similar for its watcher at https://github.com/microsoft/TypeScript/blob/1298f498f4cde7fa53cbf050ebfeefdf85c848ec/src/compiler/watchUtilities.ts#L42 , and that ts.sys.fileExists does not cache anything, so we need to do it ourselves.

Should help #1130 and #1139

I also found out during that that we are likely not properly removing Svelte snapshots when its file is deleted. I'll try this out later on.

Another possible speed-up I found was this method in completions, according to the perf analysis some time is spent in that area in total, probably due to source mapping. We could speed that up in this case because the textEdit ranges are probably all the same, so we can create a cache there. Update: I was wrong, textEdit is rarely filled especially when it's about global completions. But I found that we can compute path once only because pathToUrl is not that fast either.

@jasonlyu123 thoughts?

[jasonlyu123]

Looks good. The only downside I could think of it's where files from outside of the workspace or inside node_module got created or deleted. But it has also been cached in module resolution cache anyway.