Adds FontImage Markup Extension for FontImageSource (#6398)

Adds FontImage Markup Extension for FontImageSource (#6398)

* Added FontImage markup extension which simplifies usage of FontImageSource into any ImageSource

* Added unit tests to cover TabBar which is used in Shell

* Cleaned up xaml for test code

* Removed class references from unit test project which were added by mistake

* Removed FontImageExtension from markup parser

* Added ContentProperty to markup extension

* Added Size to FontImageExtension

* Updated unit tests for new properties

* added unit tests to certify each property was set correctly by the markup extension

* Added content property test to certify that Glyph is being set correctly

* Flipped order in unit tests of expected and actual

* Updated default value for Size to use the FontImageSource default's value. This will keep it in sync if that value ever changes

fixes #6376