Add Steepfile as Ruby filename

[mattt]

steep is a static type checker for Ruby based on RBS.

Users can run the steep check command to type check their Ruby code. This command looks for a file named Steepfile, a Ruby file that configures how type checking is performed.

Example Steepfile

target :lib do
  signature 'sig'

  check 'lib'

  configure_code_diagnostics do |config|
    config[Steep::Diagnostic::Ruby::UnsupportedSyntax] = :hint
  end
end

target :test do
  signature 'sig'

  check 'spec'
end

According to GitHub code search there are currently a few hundred instances of Steepfile in the wild. I expect this number to grow over time as developers start to adopt RBS in their Ruby codebases.

Description

This PR adds Steepfile to the list of Ruby filenames and adds a sample file to samples/Ruby/filenames/Steepfile

Checklist:

• I am associating a language with a new file extension.
  • The new extension is used in hundreds of repositories on GitHub.com
    • Search results for each extension:
      • https://github.com/search?utf8=%E2%9C%93&q=filename%3ASteepfile&type=Code&ref=searchresults
  • I have included a real-world usage sample for all extensions added in this PR:
    • Sample source(s):
      • https://github.com/mattt/linguist/blob/8581d4d2a1c93ea4fa348b428c59702561a7cded/samples/Ruby/filenames/Steepfile
    • Sample license(s):
      • This sample was created by running steep init with version 0.46.0 of the steep gem, and uncommenting example code blocks. Steep is released under an MIT License, and I release this code sample into the public domain.

[lildude]

Looks like usage has a long way to go still:

Total files found: 364
Unique public user/repos: 82
Unique owners: 63

[lildude]

Ooo, I've just realised that you would generally only have one Steepfile per project so the fact we have more than 200 files would suggest it's in use by more than 200 unique :user/:repo. Approving.