Test plan for enhanced #line directives

[cston]

Proposal: dotnet/csharplang#4747
Spec: https://github.com/dotnet/csharplang/blob/main/proposals/csharp-10.0/enhanced-line-directives.md

#line (startLine, startChar) - (endLine, endChar) charOffset "fileName"

Parsing

• -langversion
• in inactive region
• incomplete directive
• missing tokens
• unexpected token types
• with extra/no trivia between tokens

Semantics

• CSharpSyntaxTree.GetLineMappings()
• values out of range
• (endLine, endChar) < (startLine, startChar)
• before/after #line default, #line hidden, #line number
• diagnostic reported for mapped span
• sequence points in mapped span
• diagnostics and GetLineMappings() return default (unmapped) spans after malformed #line
• spans that start/end before/after charOffset with single- and multi-line spans
• spans shorter/longer than mapped span with single- and multi-line spans
• SyntaxNormalizer
• CSharpCompiler.ResolveEmbeddedFilesFromExternalSourceDirectives()

Productivity

• Classification: ClassifyTrivia(), ClassifyPreprocessorDirective()
• Formatting
• EnC after changing #line
• CSharpSyntaxFacts.TryGetExternalSourceInfo()

[cston]

Closing test plan issue. We can open separate issues if other issues are discovered.