Add optional fa_icon_pack property to social_links to allow more Font Awesome icons packs beyond brands

[pfirpfel]

Summary

Extends functionality introduced in #839. Proposal for my issue raised here: #843

Interface changes

Adds an optional property to the config keys under minima:social_links. Will not break existing configurations.

Motivation

Allow users to use icons from Font Awesome beyond the brands icon pack in their social links.

[ashmaroli]

Hello again, @pfirpfel
I am not entirely in favor of using Liquid's default filter in a for loop owing to potential performance issues with using the theme in a site with numerous posts and pages because of this conditional flow.
(imagine calling :respond_to? for every social-icon entry with fa_icon_pack property set to a non-falsey value for every post / page in the site.. will it be too much of a concern? Maybe, maybe not.)

The one viable solution I see here is to force everyone to set the entire necessary Font Awesome icon classlist:

minima:
  social_links:
    - title: Jekyll repository at GitHub
      icon: "fab fa-github"
      url: "https://github.com/jekyll/jekyll"
    - title: Email Jekyll
      icon: "far fa-envelope"
      url: "mailto:jekyll@example.com"

Note how one can now opt to use fab or far instead of the expanded forms fa-brands and fa-regular respectively if they are familiar with that syntax.

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N.B. You need not rebase your branch if you choose to make changes. We use squash merges here.