Add --with-conditional to wrap single-file output in a preprocessor conditional

[tannergooding]

There's no way today to wrap a generated file in a preprocessor conditional. --headerFile prepends a fixed header, but nothing emits a leading #if/trailing #endif.

This adds --with-conditional <symbol> / -wcond which wraps single-file C# output in a leading #if <symbol> and a trailing #endif:

#if VERSION_2_OR_NEWER

namespace ClangSharp.Test
{
    public partial struct MyStruct
    {
        public int x;
    }
}
#endif

The motivating scenario (#365) is Unity, where files can't be conditionally excluded at the project level, so the consumer generates each struct variant separately and selects one via a build symbol. XML output is intentionally unaffected.

---

Scoped to single-file C# output on purpose, which is exactly the run-once-per-variant flow in the issue. Wrapping multi-file output is a larger design — you'd want the symbol to vary per file (a remap), where-as a single global symbol is trivial. Left as a follow-up if someone actually needs the multi-file variant.

Adds a golden test (ConditionalOutputTest).

Fixes #365