Escape assembly name when starting with number

[Manodasan Wignarajah (manodasanW)]

Fixes #1880

[Copilot]

Pull Request Overview

This PR ensures that assembly names beginning with a digit are prefixed with an underscore before sanitizing, preventing invalid identifiers.

• Prefixes an underscore when the input starts with a numeric character
• Continues to replace all non-alphanumeric/underscore characters with underscores

Comments suppressed due to low confidence (1)

src/Authoring/WinRT.SourceGenerator/Helper.cs:1241

• Add a unit test for inputs starting with a digit (e.g., "1MyLib") to verify the underscore prefixing behavior is applied correctly.

            var prefixedTypeName = Regex.Match(typeName, @"(^\d)").Success ? "_" + typeName : typeName;