`StringSyntax` support for VB and VB-test in C# strings

Summary

Support semantic classification for strings annotated with the StringSyntax attribute with VB and VB-test

Background and Motivation

When working with Roslyn, string literals are often used to denote source files. C#-test is already present which also handles Roslyn-specific markup, and C# is something that is also being supported although not yet present in VS.

Proposed Feature

The proposal allows the user to use VB or VB-test as valid identifiers in StringSyntax attributes, and Roslyn to offer semantic classification for strings annotated with these language identifiers. This feature will be valid at least for C#, where the semantic classifier will respect and properly classify string literals that are annotated with any of the above VB identifiers.

For example consider this C# source,

// lang=vb-test
const string source = """
    Imports System

    Public Sub Method()
        Const greetings As String = "Hello World!"
        Console.WriteLine(greetings)
    End Sub
    """;

In the string literal itself, since it's annotated with lang=vb-test, Roslyn will offer semantic classification for the source as a VB source, thus enabling IDEs to highlight parts of the string literal accordingly.

Alternative Designs

None.

Start by locating the existing StringSyntax support for the C#-test identifier and the semantic classifier that handles annotated C# string literals. Trace how language identifiers are validated and how embedded source is classified. Done means VB and VB-test are accepted in StringSyntax annotations and the example VB source receives semantic classification.