Make Generated Classes for TextAlignmentExtensions for .NET MAUI Controls internal

[brminnick]

Description of Change

This PR mirrors the similar recently merged PR in CommunityToolkit.Maui, CommunityToolkit/Maui#791

• Adds the Unit Tests added in Make TextColorTo Generated Classes for .NET MAUI Controls Internal Maui#791
• Changes the default class accessor for code generated
  • .NET MAUI Controls: internal
  • User Created Controls: Match the provided class accessor
    • E.g. public class MyControl : ITextStyle will generate public static partial class TextAlignmentExtensions_MyControl
    • E.g. internal class MyControl : ITextStyle will generate internal static partial class TextAlignmentExtensions_MyControl

Linked Issues

• Fixes [BUG] Source generator error - The call is ambiguous between the following methods or properties - CommunityToolkit.Maui.Markup.TextAlignmentExtensions #158

PR Checklist

• Has a linked Issue, and the Issue has been approved(bug) or Championed (feature/proposal)
• Has tests (if omitted, state reason in description)
• Has samples (if omitted, state reason in description)
• Rebased on top of main at time of PR
• Changes adhere to coding standard

[pictos]

LGTM!