parser, checker: check invalid lambda expr (fix #20414)

[yuyi98]

This PR check invalid lambda expr (fix #20414).

• Check invalid lambda expr.
• Add test.

fn main() {
	x := 5
	f := if x == 5 { |x| x - 5 } else { |x| x }
	println(f(x))
}

PS D:\Test\v\tt1> v run .
tt1.v:3:19: error: lambda expressions are allowed only in places expecting function callbacks
    1 | fn main() {
    2 |     x := 5
    3 |     f := if x == 5 { |x| x - 5 } else { |x| x }
      |                      ~~~~~~~~~
    4 |     println(f(x))
    5 | }

[spytheman]

Excellent work.

[zeozeozeo]

I think this should be allowed in the future, lambdas should act as functions.

For example, this is allowed:

import term

fn print_colored(is_red bool, text string) {
    color := if is_red { term.red } else { term.white }
    print(color(text))
}

but this is not:

import term

fn print_colored(is_red bool, text string) {
    color := if is_red { |text| term.red(text) } else { |text| term.white(text) }
    print(color(text))
}