fix type method on function values

[zolamk]

Describe the bug

Currently for the FunctionValue type the Type() method only has a placeholder implementation which is fine for now since it's not in use, we need to have a correct implementation that returns the correct function signature.

reviewpad/lang/value.go

Lines 221 to 225 in c5a4d1a

	// TODO: Implement a more reliable function type
	// that returns the actual parameters and return type
	func (fVal *FunctionValue) Type() Type {
	return BuildFunctionType([]Type{}, BuildBoolType())
	}