Assert that a type is a function.
Type that must be callable
Accepts callable types including function declarations and arrow function types.
type A = AssertFunction< () => void >; // ✓type B = AssertFunction< {} >; // ✗ TS error Copy
type A = AssertFunction< () => void >; // ✓type B = AssertFunction< {} >; // ✗ TS error
Assert that a type is a function.