Allow to set default includePath and browse.path in settings.json

[rongjiecomputer]

Problem:

Currently includePath and browse.path can only be set in project-specific .vscode/c_cpp_properties.json and this leads to a lot of problems:

1. Have to copy the c_cpp_properties.json in every project.
2. Single file cannot have c_cpp_properties.json, so no intelliSense at all.
3. A git repo cannot share c_cpp_properties.json because some users have very different includePath and browse.path due to non-default header/library installation.

Feature request:

Allow to set default includePath and browse.path in VSCode settings.json.

If includePath and browse.path are set in both settings.json and project-specific c_cpp_properties.json, includePath and browse.path in c_cpp_properties.json should have higher precedence, just like -I and /I flags have higher precedence than -isysroot flag and INCLUDE environment variable.

It might also make sense to have a separate symbol database for includePath and browse.path from settings.json to avoid re-parsing.

[sean-mcmanus]

Yes, we were already planning to do this. ETA maybe February (not sure yet). I think we're tracking this via #368 .