Suggest valid target keys for common typos

[aouxwoux]

what changed

when someone writes link-flags, compile-flags, or flags, cmkr now suggests the config key that actually exists.

the suggestion is only shown when that key is valid for the current target type.

why

these are pretty natural names to try, and the old error did not give much to go on.

checks

• git diff --check passes.
• github ci passed on the previous revision; the amended commit will rerun it.
• the local machine does not have cmake, ninja, or a c++ compiler installed.

fixes #134

[mrexodia]

I think it's a good start, but I think a levenshtein distance between existing keys could also help. For example if you type cmpile-flags it would suggest compile-flags