Add scripts/validate-flags.sh

[straight-shoota]

This script extracts all flag? macro values from the source code and compares them with the documentation.
It lists all flags that are currently defined but undocumented.

It depends on crystal tool flags, proposed in crystal-lang/crystal#14234

This could be automated in CI to check flag documentation regularly.
Adds a CI workflow that regularly checks flag documentation coverage.

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[straight-shoota]

As @Blacksmoke16 observed in #735 (comment), this script did miss out on flags that are handled by the compiler directly.

I added that now. It's not a fancy compiler tool that investigates the AST. But crystal tool flags is useful for any Crystal program, whereas Program#has_flag? is only relevant for the compiler. So a crude grep implementation does the job as well. There's less variation because the method only accepts String. And we control all the code.