Project cache and cache clean-up

[hinell]

• https://github.com/microsoft/vscode/issues/32461

VSCode users experience serious issues with storage space wasted because of growing cache size in huge C++ projects.

I propose to localize critical clangd cache data to .vscode/ folder local to the project so that it travels along with it if project is moved or global vscode editor workspace cache gets kiled or corrupted.

There are similiar issues with MS cpp-tools:

• https://github.com/microsoft/vscode-cpptools/issues/3664
• https://github.com/microsoft/vscode-cpptools/issues/3762