feat: wp-280 use @mixin, deprecate @extend

[wesleyboar]

Overview

Use @mixin. Deprecate @extend.

Status

• mixins:
  • [x] message coupled with c-message
  • [x] message de-coupled from c-message
  • [ ] forms using c-message
  • [x] truncate, untruncate
  • [x] link
  • [ ] tabs
  • [ ] layout
  • [ ] tabs
  • [ ] grid
  • [ ] figure
  • [ ] center
  • [ ] blockquote
  • [x] article-link
• review:
  • [ ] author "Testing"
  • [ ] author "UI"
• clients:
  • [ ] test some backwards compatibility
  • [ ] review updates necessary to abandon @extend

Related

• WP-280

• 340

• requires #347
• fixes TUP-727

Changes / Testing / UI

• 327

• 332

• 335

• 336

• 339

[wesleyboar]

Update: postcss-mixins is installed in main (since #347), so now this PR is solely replacing extend with mixin, to resolve WP-280 while being backwward-compatible.

[wesleyboar]

This is needed to fix TUP-727. No need to get everything all at once. Just make sure what is in works. #336 seems to make the existing set reliable. I opt to fall-forward[^1] with this feature, because it is very important I stop using @extend now, because bugs are coming and log warnings and console errors are many.

[^1]: Deploy with high confidence. Fix if something comes up.