Rewrite or clean up settings migrator

[jakebailey]

There's a settings migrator in updateTestSettings.ts; it's name is misleading as this handles quite a few setting migrations. The migrator works by directly modifying the settings files, using heuristics and regex to attempt to fix the settings. This is error prone and has led to bugs in the past; we can't even use the jsonc library to edit the file safely as the library itself is buggy when removing data.

VS Code itself has the ability to safely modify settings; it'd be nice for this migrator to be rewritten to use the VS Code APIs to edit the settings directly.