FontIconSource markup extension

[Sergio0694]

This PR is part of the group being tracked in #3108.

PR Type

What kind of change does this PR introduce?

• Feature

What is the new behavior?

A XAML markup extension that creates a FontIconSource from a string representing a Segoe MDL2 icon, resulting in much cleaner code (see here). All additional info can be found in the linked issue.

This issue is related to #3110.

PR Checklist

Please check if your PR fulfills the following requirements:

• Tested code with current supported SDKs
• Pull Request has been submitted to the documentation repository instructions. Link:
• Sample in sample app has been added / updated (for bug fixes / features)
  • Icon has been created (if new sample) following the Thumbnail Style Guide and templates
• Tests for the changes have been added (for bug fixes / features) (if applicable)
• Header has been added to all new source files (run build/UpdateHeaders.bat)
• Contains NO breaking changes

[ghost]

Thanks Sergio0694 for opening a Pull Request! The reviewers will test the PR and highlight if there is any conflict or changes required. If the PR is approved we will proceed to merge the pull request 🙌

[vgromfeld]

[michael-hawker]

@Sergio0694 I feel like this should this just be added to the #3110 PR instead? (And we should have tests for both.)

[michael-hawker]

@Sergio0694 can we just merge this into the PR for #3110 since it provide very similar functionality? Thanks!