Improve project root detection in LSP

[edemaine]

This is an attempt at #1094. It seems to work better in development, but there are some aspects specific to dev vs. actual extension that might differ.

• Populate dist/lib with a tsconfig.json (from new source/tsconfig.json). This helps the LSP figure out that the provided lib files are in their own project root.
• Detect project roots in more ways, to handle cases with no tsconfig.json:
  • If we're in a node_modules/foo directory, use that. This takes priority over tsconfig.json because findConfigFile searches in all ancestor directories, which could go very far up.
  • Fallback to source project directory only if the opened file is inside that directory. Otherwise, fallback to the directory containing the file (isolation).

I'm still getting the following errors about duplicate definitions. I'm not quite sure why; it seems like TypeScript is loading some libs despite my asking it not to? But at least it's only seven errors, and it seems isolated to the .d.ts file.