v: allow comptime-for to iterate over comptime variables + add $string comptime type + cleanup

[felipensp]

This PR allow to iterate on compile-time variables (related to #20176):

• Feature: $for t in var.variants {
• Add $string compile-time type
• Cleanup ast.Table

type TestSum = int | string

struct Bar {
    y int
}

struct Foo {
mut:
	a TestSum
    b int   
    c string
    d Bar
}

fn main() {
    $for f in Foo.fields {
        $if f.typ is $sumtype {
            dump(f)
            $for f2 in f.variants {
                dump(f2)
                $if f2.typ is $int {
                    dump('int')
                } $else $if f2.typ is $string {
                    dump('string')
                }
            }
        } $else $if f.typ is Bar {
            $for f3 in f.fields {
                println(f3)
            }
        }
    }
}

copilot:summary

copilot:walkthrough

[spytheman]

Wow, I could have swear, that string was here already. Good addition.

[spytheman]

Excellent work.