NOTE: This is committed to the 3.3 branch as part of D15606, which we
decided should go to this release still (by Bastien, Dalai and me). That
is because these are important usability fixes/improvements to have for
the LTS release.

Adds `rna_path.cc` and `RNA_path.h`.

`rna_access.c` is a quite big file, which makes it rather hard and
inconvenient to navigate. RNA path functions form a nicely coherent unit
that can stand well on it's own, so it makes sense to split them off to
mitigate the problem. Moreover, I was looking into refactoring the quite
convoluted/overloaded `rna_path_parse()`, and found that some C++
features may help greatly with that. So having that code compile in C++
would be helpful to attempt that.

Differential Revision: https://developer.blender.org/D15540

Reviewed by: Brecht Van Lommel, Campbell Barton, Bastien Montagne