Support `--stdin-path` or something similar to properly ignore files formatted through stdin

[reese]

This is more a feature request since I imagine other tools (e.g. VSCode extensions will need it, see the discussion in #354) will need it: currently, any tool formatting through stdin loses all ability to use .rubyfmtignore, but there are plenty of cases where reading from disk isn't convenient/possible, especially in VSCode extensions like the Ruby and Sorbet extensions.

Having something like --stdin-path from prettier would simplify this problem by basically telling rubyfmt to format file contents as if they were at the given path, and ignore them as needed. This prevents extensions from having to manually handle .rubyfmtignore paths.

So for example, if my .rubyfmtignore has foo.rb in it, doing

echo 'puts "hi"' | rubyfmt --stdin-path foo.rb
# => puts "hi"

produces no changes due to the file being ignored, but

echo 'puts "hi"' | rubyfmt --stdin-path bar.rb
# => puts("hi")

produces changes as expected.

[reese]

Hmm, so there's a bit of a snag on this, which is mainly that BurntSushi/ripgrep#829 is still kicking around, which affects exact use-case. We're also not the first Rust-based code formatter (see JohnnyMorganz/StyLua#377) to get bitten by this, so I think this will likely turn out to be non-trivial (i.e. it may require hand-rolling out ignore logic, which sounds not-fun).

Gonna put this on the backburner for a bit, but it's worth noting that this also means that some pretty basic ignore patterns are gonna be busted for users, so this makes the .rubyfmtignore and .gitignore checking a bit less useful.