Adds very basic CSS support for Border

[sthewissen]

Description of Change

Added basic support for CSS properties that work on the Border control. The following properties are now supported:

.myNicePurpleBorder
{
   border-color: #512BD4; /* Also supports linear-gradient, radial-gradient, named colors etc. */
   border-radius: 10; /* Always maps to RoundRectangle [Radius] internally */
   border-width: 4;
}

Issues Fixed

Partially fixes #27296 to get basic support going, making it become on par with existing support for Frame, Button and ImageButton. It could definitely be expanded upon in the future for Border explicitly, but a discussion is to be had on how that would translate to missing features on the other controls that support borders.

[dotnet-policy-service]

Hey there @sthewissen! Thank you so much for your PR! Someone from the team will get assigned to your PR shortly and we'll get it reviewed.

[rmarinho]

/rebase

[jsuarezruiz]

/azp run

[azure-pipelines]

Azure Pipelines successfully started running 3 pipeline(s).

[bricefriha]

Much needed, thanks @sthewissen!