checker: check generic sumtype declaration (fix #18741)

[yuyi98]

This PR check generic sumtype declaration (fix #18741).

• Check generic sumtype declaration.
• Add test.

type Fn[T] = fn (string) T

type Fnn = fn (string) string

type Parser = Fn | Fnn

fn main() {
	println('hello')
}

PS D:\Test\v\tt1> v run .
tt1.v:5:15: error: generic fntype `Fn` must specify generic type names, e.g. Fn[T]
    3 | type Fnn = fn (string) string
    4 | 
    5 | type Parser = Fn | Fnn
      |               ~~
    6 | 
    7 | fn main() {

type Fn[T] = fn (string) T

type Fnn = fn (string) string

type Parser[T] = Fn[T] | Fnn

fn f(x string) string { return '>> f $x' }

fn main() {
    _ := Parser(f)
}

PS D:\Test\v\tt1> v run .
tt1.v:10:10: error: generic sumtype `Parser` must specify type parameter, e.g. Parser[int]
    8 | 
    9 | fn main() {
   10 |     _ := Parser(f)
      |          ~~~~~~~~~
   11 | }

[spytheman]

type Fn[T] = fn (string) T

type Fnn = fn (string) string

type Parser[T] = Fn[T] | Fnn

fn f(x string) string { return '>> f $x' }

fn main() {
    _ := Parser(f)
}

has a cgen error too.

_ := Parser[string](f) does compile however.

Imho the Parser(f) expression should either produce a checker error that the type can not be inferred, or it should infer it by the type of f, i.e. that it is string.

I am not sure what will be more maintainable, since in this case both Fn[T] and Fnn are compatible with the type signature for f . Perhaps it would be just easier/simpler to require the type name explicitly.