Introduce .flex-column-reverse utility

[simurai]

This adds the .flex-column-reverse utilities to complete the rest of the directional classes.

TODO

• Add utility
• Add documentation

Size changes

┌─────────────────────┬───────────┬─────┬───────────┬────────┬──────────┬─────────┬──────────────────────────────┐
│ name                │ selectors │   ± │ gzip size │      ± │ raw size │       ± │ path                         │
├─────────────────────┼───────────┼─────┼───────────┼────────┼──────────┼─────────┼──────────────────────────────┤
│ primer              │      3290 │ + 5 │    26.3 K │ + 24 B │ 165.73 K │ + 322 B │ dist/primer.css              │
│ core                │      2206 │ + 5 │   17.52 K │ + 23 B │  111.2 K │ + 322 B │ dist/core.css                │
│ utilities           │      1357 │ + 5 │    8.61 K │ + 24 B │  65.55 K │ + 322 B │ dist/utilities.css           │

Closes #915

/cc @primer/ds-core

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[shawnbot]

This is great, thanks! I've changed the base to #925 since we'll probably (hopefully!) get around to that before another minor release.