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