Drop ActionMailer as a dependency

[coorasse]

I recently setup solid_errors in a rails minimal app. When rails is setup with --minimal ActionMailer is not required (i.e. require "action_mailer/railtie" is commented out).

This causes problems when Zeitwerk eager loads stuff:

/.../vendor/bundle/ruby/3.4.0/gems/solid_errors-0.6.1/app/mailers/solid_errors/error_mailer.rb:3:in '<module:SolidErrors>': uninitialized constant SolidErrors::ActionMailer (NameError)
Did you mean?  ActionView
from /.../vendor/bundle/ruby/3.4.0/gems/solid_errors-0.6.1/app/mailers/solid_errors/error_mailer.rb:1:in '<top (required)>'

Pity, because this is really a minimal rails app, and I configured config.solid_errors.send_emails = false.

Shall I work on a PR?

[fractaledmind]

Good point. Given that the engine can be configured to not send emails, Action aller should not be a hard dependency. A PR would be perfect