RFC: a proposal for input validation

[MathieuDuponchelle]

As I think input validation and generating the reference manual (as opposed to hand-writing and maintaining it in a standalone markdown file) could go hand in hand, I've taken some time to design a solution and write a simple POC for it, which I pushed at https://github.com/MathieuDuponchelle/meson-input-validation-poc, as I think some code is the best way to kickstart the discussion.

The discussion points I could think of are addressed in the README, along with an explanation of the proposed approach.

Curious what you smart people think :)