Horizontal direction specific class names set in variables

[pau1phi11ips]

Allows start/end class names to be overridden in a build step to ease the migration from Bootstrap 4 to Bootstrap 5.

Motivation & Context

An old project wouldn't have to rename every *-left, *-right, text-align-*, ml*, mr*, pl*, pr* class in a codebase.
Settings to use the Bootstrap 4 class names:

$start:                      left;
$s:                          l;
$end:                        right;
$e:                          r;

Type of changes

• Refactoring (non-breaking change)

Checklist

• I have read the contributing guidelines
• My code follows the code style of the project (using npm run lint)
• N/A My change introduces changes to the documentation
• N/A I have updated the documentation accordingly
• N/A I have added tests to cover my changes
• All new and existing tests passed

Live previews

No changes to end CSS

Related issues

None

[pau1phi11ips]

Maybe the small version of start/end should just be $s and $e?