Add PhutilRemarkupHexColorCodeRule, a new remarkup rule to format color codes

Summary:
Implements a remarkup rule that formats the background color to match the value of inline hex color codes.

The format for this rule looks like {#RRGGBB} and it will be formatted the same as monospace text, so #RRGGBB with the background set to the specified color.

Test Plan:

• run arc unit and see the tests pass
• create some markup with {#ff0000} color codes for all of your favorite colors {#ee33ee}

Reviewers: O1 Blessed Committers, avivey

Reviewed By: O1 Blessed Committers, avivey

Subscribers: avivey, aklapper, tobiaswiese, valerio.bozzolan, Matthew, Cigaryno

Maniphest Tasks: T15744

Differential Revision: https://we.phorge.it/D25540