It's not a thing and I totally agree it should exist, there's a proposal for it on GitHub.
If you want to handle different types, the right way of doing it is giving your parameter a generic type then checking what it is in the function.
func _ready():
handle_stuff(10)
handle_stuff("Hello")
func handle_stuff(x: Variant):
if x is int:
print("%d is an integer" % x)
elif x is String:
print("%s is a string" % x)
This prints 10 is an integer and Hello is a string.
If you really, really need to have a variable amount of arguments in your function, you can pass an array. It's pretty inefficient but you can get away with it.
func handle_stuff(stuff: Array):
for x: Variant in stuff:
if x is int:
print("%d is an integer" % x)
elif x is String:
print("%s is a string" % x)
Then you can pass [10, 20, 30] into it or something. It's a useful trick.