Add to_list() method to include_directories objects

[gabeblack]

This adds a to_list() method on the inc object returned by include_directories. The to_list() method returns a list of strings, where each string is an absolute path (one for the path relative to the source root and the other one for the path relative to build root). If the paths are system paths, then a single path is returned (as-is).

This addition was discussed in this thread: https://github.com/mesonbuild/meson/issues/12012#issue-1811035658

[bruchar1]

Could you please squash all your commits into one?

[gabeblack]

Could you please squash all your commits into one?

done.

[gabeblack]

Reopening -- I didn't mean to close the PR.

[gabeblack]

@bruchar1 @jpakkane -- what are the next steps to get the approved/merged?

[gabeblack]

@bruchar1 @jpakkane -- Are contributions outside of the meson team discouraged? I'm ignorant as to your processes here and am wondering why the radio-silence?

[gabeblack]

Was informed that meson can take multiple-months to look at or respond to questions in a PR. Unfortunately, my current work/life balance doesn't allow for me to maintain outside work for long dev cycles, so closing this PR as to not clutter up the PR backlog and make the dev cycle longer for others that are able to do so.