MWPW-153372 - Add ability to further customize buttons

[elan-tbx]

• After some discussion, a pattern has been agreed upon for extending our button styles beyond the base 3 (text link, outlined button, filled button). For now, the main variant that this PR enables is the "black-filled" style, which already had preexisting CSS written for it in styles.css, but it opens the door for additional customizations in the future.
• The authoring pattern for this is simply adding #_button-{style} to the end of the button's URL. In the case of the black-filled button, for example, it would be #_button-fill. Whatever is in the style part of the tag will be applied as a CSS class to the button, via the decorateButtons() function.
• Note that this should not interfere with other link customizations such as #_dnt or modal links -- multiple tags can be added to a single link, and there is documented precedent for doing this already in Milo (see the "Link Features" tab of this authoring documentation)

Resolves: MWPW-153372

Test URLs:

• Before: https://main--milo--adobecom.hlx.page/drafts/ebartholomew/buttons?martech=off
• After: https://ebartholomew-MWPW-153372-custom-btns--milo--adobecom.hlx.page/drafts/ebartholomew/buttons?martech=off

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Commits

• 0c8cfae ✅ (latest)
• 1301d5e ✅
• b1a3948 ✅
• 5950658 ✅
• 851d5f1 ✅
• 6f8f6bc ✅
• d14fabe ✅
• 94442f3 ✅

[aem-code-sync]

	Page	Scores	Audits	Google
M	/drafts/ebartholomew/buttons?martech=off
D	/drafts/ebartholomew/buttons?martech=off

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.91%. Comparing base (0ae86a3) to head (0c8cfae).
Report is 17 commits behind head on stage.

Additional details and impacted files

@@            Coverage Diff             @@
##            stage    #2776      +/-   ##
==========================================
+ Coverage   95.89%   95.91%   +0.02%     
==========================================
  Files         173      173              
  Lines       45853    45852       -1     
==========================================
+ Hits        43969    43979      +10     
+ Misses       1884     1873      -11     

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[ryanmparrish]

Amen 🙏

[github-actions]

This PR is currently in the needs-verification state. Please assign a QA engineer to verify the changes.

[SilviuLCF]

Verified , ready to STAGE
Testing details: https://jira.corp.adobe.com/browse/MWPW-153372