cgen: fix option map with fn type value (fix #18786)

[yuyi98]

This PR fix option map with fn type value (fix #18786).

• Fix option map with fn type value.
• Add test.

const mouse_action_to_function_map = {
	1: handle_mouse_down_signal
}

fn handle_mouse_down_signal() string {
	return 'hello'
}

fn main() {
	t := mouse_action_to_function_map[1] or { return }
	println(t())
	assert t() == 'hello'
}

PS D:\Test\v\tt1> v run .
hello