parser: check non-generic interface defining generic method (fix #20430)

[yuyi98]

This PR check non-generic interface defining generic method (fix #20430).

• Check non-generic interface defining generic method.
• Add test.

module main

interface Abc {
	test[T]()
}

struct Xyz {}

fn (xyz Xyz) test[T]() {}

fn main() {
	_ := Abc(Xyz{})
}

PS D:\Test\v\tt1> v run .
tt1.v:4:6: error: non-generic interface `Abc` cannot define a generic method
    2 | 
    3 | interface Abc {
    4 |     test[T]()
      |         ^
    5 | }
    6 |

[spytheman]

Excellent work.

[spytheman]

I am curious, why use interface Expr<R> {, instead of the newer interface Expr[R] { ?

[yuyi98]

Ok, I'll cleanup them.