Added Msix.assetUri() to get an ms-appx:/// URI of Flutter assets

[Levi-Lesches]

• Adds Uri Msix.assetUri(String)
• Adds section to the README on how to get ms-appx URIs for Flutter assets
  • Since users may now be importing package:msix, I removed the recommendation to use a --dev dependency
• Adds test to make sure the new function works as expected
• Bumps pubspec version and updates changelog (minor version)

Closes #291.

Consider merging #294 instead.