Allow additional stylesheets to be included in the admin UI

[tvdeyen]

What is this pull request for?

Adds Alchemy.admin_styleheets to set additional stylesheets to be included in the Alchemy admin UI.

Example

# config/initializers/alchemy.rb
Alchemy.admin_stylesheets << "alchemy/devise/admin.css"

Checklist

• I have followed Pull Request guidelines
• I have added a detailed description into each commit message
• I have added tests to cover this change

[codecov]

Codecov Report

Attention: Patch coverage is 87.50000% with 1 line in your changes missing coverage. Please review.

Project coverage is 96.59%. Comparing base (010f055) to head (fe14790).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/alchemy/engine.rb	83.33%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3179   +/-   ##
=======================================
  Coverage   96.58%   96.59%           
=======================================
  Files         236      236           
  Lines        6360     6366    +6     
=======================================
+ Hits         6143     6149    +6     
  Misses        217      217           

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