Generate rbi for mailer classes

[hdoan741]

For each instance mailing method, we detect the signature and generate the corresponding class method.

• If a method has a sig, we'll generate a method with signature based on Sorbet's inflection data
• If a method doesn't have one, we'll generate a method with parameters all untyped
• The return type of a class method is ActionMailer::MessageDelivery, so that you can call "deliver_now", "deliver_later" on it.

[codecov-io]

Codecov Report

Merging #151 into master will increase coverage by 0.11%.
The diff coverage is 99.47%.

@@            Coverage Diff             @@
##           master     #151      +/-   ##
==========================================
+ Coverage   98.24%   98.36%   +0.11%     
==========================================
  Files         153      169      +16     
  Lines        1650     1830     +180     
==========================================
+ Hits         1621     1800     +179     
- Misses         29       30       +1

Impacted Files	Coverage Δ
spec/mailer_rbi_formatter_spec.rb	100% <100%> (ø)
...c/support/v5.1/app/mailers/daily_prophet_mailer.rb	100% <100%> (ø)
lib/sorbet-rails/tasks/rails_rbi.rake	93.33% <100%> (+1.14%)	⬆️
...ort/v4.2/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)
...c/support/v6.0/app/mailers/daily_prophet_mailer.rb	100% <100%> (ø)
...ort/v5.0/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)
...ort/v6.0/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)
...ort/v5.1/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)
...c/support/v4.2/app/mailers/daily_prophet_mailer.rb	100% <100%> (ø)
...v5.0/config/initializers/new_framework_defaults.rb	100% <100%> (ø)	⬆️
... and 25 more

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[jaredbeck]

Very cool, Harry! Can't wait to try it out.

Reading through the changes here was very educational for me, learning about how sorbet-rails is organized.

I have a feeling that the ability to reflect on method signatures will come in handy later.