parser,fmt: inform about invalid interop function bodies instead of removing them

[ttytm]

Fixes #12966

Where formatting

fn JS.state_is_ready_on_leave(retval int) {
	println('Hooked callback')
}

results in

fn JS.state_is_ready_on_leave(retval int)

With the changes it would throw an error instead

error: interop functions cannot have a body
    1 | fn JS.state_is_ready_on_leave(retval int) {
      |                                           ^
    2 |     println('Hooked callback')
    3 | }