Add NES013 analyzer diagnostic for throw statements

[Copilot]

The transpiler rejects Throw/Rethrow IL opcodes at transpile time, but there was no Roslyn analyzer to surface this in the IDE. Users only discovered the error during MSBuild transpilation.

Adds NES013 diagnostic for ThrowStatement and ThrowExpression syntax kinds, matching the existing pattern (e.g. NES011 for try/catch).

• Analyzer: New NES013Rule descriptor, registered for both SyntaxKind.ThrowStatement and SyntaxKind.ThrowExpression
• Release tracking: Added to AnalyzerReleases.Unshipped.md
• Tests: Three cases — throw statement, throw expression, and negative (no throw)

// NES013 error on both forms:
throw null;                                        // ThrowStatement
byte x = condition ? (byte)1 : throw null;         // ThrowExpression