Fix Visual Basic file reading examples: remove hardcoded paths, add RichTextBox support, and include project file

[Copilot]

The Visual Basic documentation for reading text files contained hardcoded absolute file paths (like "C:\test.txt") that cause issues on different systems and don't follow best practices. Additionally, users were requesting examples showing how to load text files into RichTextBox controls, which wasn't covered.

Changes Made

Fixed hardcoded file paths:

• Changed "C:\test.txt" to "test.txt" in existing code snippets to use relative paths
• This prevents issues with drive letters and path separators on different operating systems

Added RichTextBox integration examples:

• New snippet showing basic file-to-RichTextBox loading pattern
• Advanced snippet with proper error handling and Path.Combine for robust path construction

Enhanced documentation:

• Added new section "To read from a text file into a RichTextBox control"
• Included warning note about avoiding hardcoded absolute paths
• Provided guidance on using Path.Combine for cross-platform compatibility

Added project file for CI build:

• Created VbFileIORead.vbproj targeting .NET Framework 4.8 to enable proper CI builds
• Project builds successfully as a Library project

Example of the improvement:

Before:

fileReader = My.Computer.FileSystem.ReadAllText("C:\test.txt")

After:

fileReader = My.Computer.FileSystem.ReadAllText("test.txt")
' Or with proper path handling:
Dim filePath As String = System.IO.Path.Combine(System.AppDomain.CurrentDomain.BaseDirectory, "test.txt")
fileReader = My.Computer.FileSystem.ReadAllText(filePath)

These changes address user frustrations with compilation errors and provide the RichTextBox functionality specifically requested in the issue.

Fixes #33919.

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Internal previews

📄 File	🔗 Preview link
docs/visual-basic/developing-apps/programming/drives-directories-files/how-to-read-from-text-files.md	How to: Read From Text Files in Visual Basic

[BillWagner]

@copilot Can you adda vbproj file to this folder for the CI build.

[Copilot]

@copilot Can you adda vbproj file to this folder for the CI build.

Added VbFileIORead.vbproj file targeting .NET Framework 4.8. The project builds successfully as a Library project (commit 5e362d7).

[BillWagner]

This is ready for final review.