[DavidBakerEffendi]

Treat async as a contextual identifier in ordinary expression positions while preserving its structured modifier role for methods, local functions, lambdas, and anonymous methods.

For example, this valid C# currently produces a parse error because the argument is tokenized as a modifier instead of an identifier:

class C
{
    void M(bool async)
    {
        Use(async);
    }
}

The grammar now permits that identifier use while explicitly preferring the modifier interpretation where required. The corpus regression also keeps genuine async methods and lambdas structured.

Fixes part of #276