Add package's own inst/include with "Generate C/C++ configuration"...

[CGMossa]

Is your feature request related to a problem? Please describe.
Currently, when invoking "R package: Generate C/C++ configuration", every package
that is stated in LinkingTo is added in the includePath path via <pkg>/inst/include,
in that you run something akin to system.file("include", package = <pkg>).

What would be very nice, is if you tested if the current package had a inst/include, and
added that to the includePath in .vscode/c_cpp_properties.json.

I wouldn't add src/include, as that seems excessive, but it is also an option.