checker: check option propogation in multi return types

[Delta456]

Closes #18566

🤖 Generated by Copilot at f90c01d

This pull request adds a checker error for multi-assignment with optional values that are not handled or propagated. It also adds a test case for the error using the split_once method from the strings module.

🤖 Generated by Copilot at f90c01d

• Add a check for multi-assignment with optional values (link)
  • Report an error if the optional value is not handled or propagated (link, link)

[felipensp]

I think it should not be automatically propaged.

I could have intentional none values from the multi return values. And so handle it logically, not returning because some value is none.

Example:

fn get() (?int, ?f64) {
	return 1, none
}

fn main() {
	x, y := get()

	if x != none {
		println('x')
	} else {
		println('y')
	}
}