Files::get_regular_files_recursive_proximate

[ras0219-msft]

This PR implements Files::get_regular_files_recursive_proximate, which returns a list of all regular files from a subdirectory without that base path prepended. The result is the same as if you called

// pseudocode
files = Files::get_regular_files_recursive(dir);
for (auto& file : files) {
    file = file.lexically_proximate(dir);
}

However it generates the desired contents directly instead of "subtracting" after the fact on Linux.

This is needed for #296