fix: enable to select some formatters without --allow-missing-formatter

[nokazn]

Description

This PR changes checking missing formatters logic like the below.

• If some formatters are specified with -f/--formatters, check whether they are defined in config and executable.
• If not specified, check whether all formatters are executable

I think it will resolve #210.

If something goes wrong, feel free to point it out. Thank you.

[zimbatm]

thanks!

[MangoIV]

heya! this works for me and matches how I would expect it to work, however somehow this invalidates the cache; e.g.

• configure formatters a and b
• run treefmt
• run treefmt -f a
• run treefmt again, this will start reformatting with b
  see also issue what does "One or more formatters are missing" mean?  #286

[MangoIV]

the issue is that my usecase is especially that b takes horrendously long, if it invalidates the cache to run the formatter without it, I can not use treefmt to run it.