Allow extension methods in top-level programs

[aelij]

For example:

Console.WriteLine("a".EqualsInvariant("b"));

static bool EqualsInvariant(this string a, string b) =>
    string.Equals(a, b, StringComparison.InvariantCulture);

[huoyaoyuan]

Currently, functions in top level programs are treated as local functions, not ordinary functions.
This should be included in "extension method everywhere".