Add crystal tool flags

[straight-shoota]

This is a simple tool to print all macro flag? values. It's useful to compare them with the documentation (crystal-lang/crystal-book#734). Nope, they're currently not congruent (crystal-lang/crystal-book#735).

[beta-ziliani]

If this is specific to us, wouldn't it make sense to have it as a separate tool? (or even a sed script?)

[straight-shoota]

No this is not specific for the compiler/stdlib. It should be generally useful for any Crystal program that uses compile time flags.

[beta-ziliani]

The thing is that listing the flags without any explanation isn't that useful. There's the documentation for that 🤔

[straight-shoota]

Yeah, the main purpose of the tool is is to support being able to document them. It inform which flags are used so you can make sure they're all documented.

[beta-ziliani]

Right, then I don't think it belongs in the compiler

[beta-ziliani]

Scrap that last message and sorry for the confusion. This is good, although, can we have tests?

[crysbot]

This pull request has been mentioned on Crystal Forum. There might be relevant details there:

https://forum.crystal-lang.org/t/upcoming-1-12-0-release/6714/1