Make `proc-to-ast` dependency optional

[viralpraxis]

proc-to-ast is a runtime dependency for this gem. Unfortunately, it relies on parser, which does not work well with Ruby 3.4 syntax.

I propose making this dependency optional, as removing it does not cause significant harm and would simplify using this gem with modern rubies.

Currently the only usage of this gem is Proc#to_raw_source which can be enhanced with #inspect fallback.