Add Ruby linter with Steep

[lloeki]

Fixes #3254

Where are the tests? Have you added tests? Have you updated the tests? Read the
comment above and the documentation referenced in it first. Write tests!

Seriously, read :help ale-dev and write tests.

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.

[lloeki]

Been using it for a bit. Things I noticed:

• when .rbs files are changed/added/removed it seems Steep's LSP process doesn't pick the changes up, so I need to manually call :ALEStopAllLSPs. Maybe Steep needs to add some filesystem watching system to the LSP server to reload RBS definitions. Not sure if there's an automatable workaround from the ALE side.
• sometimes it runs as if there were no signatures defined. I have to figure out why (might be due to my setup).

[lloeki]

I tried to add some tests but I'm not very proficient with this whole thing, so it's taking me a bit of time.

[lloeki]

Another thing I noticed: one can have Steepfiles in a directory below where Gemfile is, and then this linter should use that instead. Indeed when there's no Steepfile at all at the top it fails.

e.g this fails:

./Gemfile
./Gemfile.lock
./gh-701/kwargs.rb
./gh-701/Steepfile
./gh-701/kwargs.rbs
./gh-708
./gh-708/foo.rb
./gh-708/Steepfile
./gh-708/foo.rbs

and this works:

./gh-701/kwargs.rb
./gh-701/Steepfile
./gh-701/Gemfile
./gh-701/Gemfile.lock
./gh-701/kwargs.rbs
./gh-708
./gh-708/foo.rb
./gh-708/Steepfile
./gh-708/Gemfile
./gh-708/Gemfile.lock
./gh-708/foo.rbs

[stale]

This pull request has been automatically marked as stale because it has not been updated recently. Make sure to write tests and document your changes. See :help ale-dev for information on writing tests.
If your pull request is good to merge, bother w0rp or another maintainer again, and get them to merge it.