3/6: Fix generating sigs for mailer with default value

[hdoan741]

Whelp, I did it. I spent some time figuring out how to parse ruby source code and extract default value for parameters!

I wasted quite a lot of time trying to parse the method signature myself, and learned that it's possible to use Regex for finding matching parenthesis! It's fascinating See: https://stackoverflow.com/questions/546433/regular-expression-to-match-balanced-parentheses#

However, the easier thing to do is using a ruby parser to do this. parser is a really nice library. I also steal some code from parlour's type parser to render a node as string easily.

The end result: we can parse the default value of optional args and optional keyword args! It's beautiful!

[codecov-io]

Codecov Report

Merging #293 into master will decrease coverage by 0.07%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   96.79%   96.71%   -0.08%     
==========================================
  Files         142      142              
  Lines        2524     2585      +61     
==========================================
+ Hits         2443     2500      +57     
- Misses         81       85       +4

Impacted Files	Coverage Δ
...pec/support/v6.0/app/mailers/application_mailer.rb	100% <ø> (ø)	⬆️
...ort/v5.0/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)	⬆️
spec/sorbet_utils_spec.rb	100% <100%> (ø)	⬆️
...ort/v6.0/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)	⬆️
...ort/v5.2/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)	⬆️
...ort/v5.1/app/mailers/hogwarts_acceptance_mailer.rb	100% <100%> (ø)	⬆️
lib/sorbet-rails/sorbet_utils.rb	93.75% <91.37%> (-3.98%)	⬇️

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[mahmoudian1]

Codecov Report

Merging #293 into master will decrease coverage by 0.07%.
The diff coverage is 94.11%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   96.79%   96.71%   -0.08%     
==========================================
  Files         142      142              
  Lines        2524     2585      +61     
==========================================
+ Hits         2443     2500      +57     
- Misses         81       85       +4     

---

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Δ = absolute <relative> (impact), ø = not affected, ? = missing data
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[codecov-io]

Codecov Report

Merging #293 into master will decrease coverage by 0.07%.
The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #293      +/-   ##
==========================================
- Coverage   96.84%   96.76%   -0.08%     
==========================================
  Files         142      142              
  Lines        2532     2598      +66     
==========================================
+ Hits         2452     2514      +62     
- Misses         80       84       +4     

Impacted Files	Coverage Δ
...pec/support/v6.0/app/mailers/application_mailer.rb	100.00% <ø> (ø)
lib/sorbet-rails/sorbet_utils.rb	94.11% <92.06%> (-3.61%)	⬇️
spec/sorbet_utils_spec.rb	100.00% <100.00%> (ø)
...ort/v5.0/app/mailers/hogwarts_acceptance_mailer.rb	100.00% <100.00%> (ø)
...ort/v5.1/app/mailers/hogwarts_acceptance_mailer.rb	100.00% <100.00%> (ø)
...ort/v5.2/app/mailers/hogwarts_acceptance_mailer.rb	100.00% <100.00%> (ø)
...ort/v6.0/app/mailers/hogwarts_acceptance_mailer.rb	100.00% <100.00%> (ø)

---

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