Add support for \e escape sequences in c# strings and characters

[JoeRobich]

See proposal for more details dotnet/csharplang#7400

[wise0704]

One thought: unknown escape sequences are simply illegal (compile time error) instead of being treated as normal string, i.e.

string text = "test \c 123"; // error CS1009: Unrecognized escape sequence

So would it be better to just treat all \. that do not match the special forms (e.g. u, U, x) as "attempted" escape characters?

[JoeRobich]

So would it be better to just treat all . that do not match the special forms (e.g. u, U, x) as "attempted" escape characters?

Sure. It would also match other grammars such as TypeScript.