Consider some way of serializing open projects in tsserver, potentially leveraging `.tsbuildinfo` files

[DanielRosenwasser]

One thing that users often hit is that file navigation might trigger opening an entire project. Opening an entire project involves

1. File loading
2. Scanning/parsing
3. Resolving dependencies
4. Keep repeating file loading on dependencies until no new files are found

This is a lot of work! If a user jumps back and forth from this file, it can re-trigger this work even if nothing has changed!

A .tsbuildinfo file is used to save time on cold compiler invocations doing this exact set of work, and to reduce work when something actually has changed. It would be interesting to see whether generating a .tsbuildinfo file after project loads could help cut down on this work.