[sourcegen] use GetText() instead of opening files from disk

[jonathanpeppers]

Description of Change

The source generator was doing:

using (var reader = File.OpenText(projItem.AdditionalText.Path))

The problem with this, is you don't actually have to hit Ctrl+S in
Visual Studio to save the files. This is potentially loading stale
files on disk.

Instead we can use:

projItem.AdditionalText.GetText().ToString();

And this will give us the string contents of the file from the
in-memory Roslyn state.

This might solve some of the weird issues in the IDE such as #4098.

However, I'm still able to reproduce #4098 after making this change.

PR Checklist

• Targets the correct branch
• Tests are passing (or failures are unrelated)
• Targets a single property for a single control (or intertwined few properties)
• Adds the property to the appropriate interface
• Avoids any changes not essential to the handler property
• Adds the mapping to the PropertyMapper in the handler
• Adds the mapping method to the WinUI, Android, iOS, and Standard aspects of the handler
• Implements the actual property updates (usually in extension methods in the Platform section of Core)
• Tags ported renderer methods with [PortHandler]
• Adds an example of the property to the sample project (MainPage)
• Adds the property to the stub class
• Implements basic property tests in DeviceTests

Does this PR touch anything that might affect accessibility?

No

[jonathanpeppers]

The diff is easier to read with ?w=1 or: