Improve Fuzzer, Drive Fuzzer Bugs to Zero, Expand Fuzzer Functions

[DanielRosenwasser]

We have a (currently) private fuzzer which simply opens OSS projects on GitHub, runs some language server operations on a project, and finds crashes. This helps pinpoint stability issues in TypeScript and the server so that we can improve reliability. This issue tracks a few things:

• Improving the fuzzer infrastructure: we believe that the codebase could benefit from some clean-up
• Expanding the fuzzer to work on TypeScript files
• Fixing crashes found by the fuzzer: there's a couple at the time of writing, and we want to drive them down to 0
• Expanding functionality of the fuzzer: we want to start performing more involved operations to proactively catch issues

[DanielRosenwasser]

I've added one bullet point around broadening the fuzzer to work on TypeScript files; it appears we only work on JS files.

[elibarzilay]

There is currently only one open issue (microsoft/tsserverfuzzer#263) which we couldn't reproduce in any way (including asking the person who replied to the linked TS issue, #39584). So I checked that box.