Introduce font family mapping

[Gillibald]

What does the pull request do?

This PR adds an option to map a family name to a different FontFamily.

.With(new FontManagerOptions
{
    FontFamilyMappings = new Dictionary<string, FontFamily>
            {
                { "Segoe UI", new FontFamily("fonts:Inter#Inter") }
            }
})
.WithInterFont()

If Segoe UI is requested the font manager provides the Inter font

What is the current behavior?

What is the updated/expected behavior with this PR?

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0052503-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]