perf(language-server): memoize possibly heavy IO utils

[cometkim]

This could fixes the LSP latency issue

I noticed that caused by mainly spawning multiple rescript -v processes on every single requests, but it could be extreamly slow if there is some process monitor (e.g. antivirus) on the environment.

Ideally, we would create a context object with the LSP server lifespan, but I added simple memoization to make fix easier.

[cometkim]

Maybe it fixes #961 too

[zth]

@cometkim great find, thank you! Here's the other part of the story - reducing latency in the analysis binary itself: #1000

This is definitely what we need to do. But, I think we need to think another lap on how and where to memoize. I'll spend some time soon to think about where to do this.

[zth]

Testing alternative approach here: #1003