Fix NodePath::slice() incorrect behavior for subname indexing

[Bromeon]

Following GDScript code:

var path = NodePath("Parent/Child:prop:subprop")
print("slice(0, 1): ", path.slice(0, 1))
print("slice(1, 2): ", path.slice(1, 2))
print("slice(2, 3): ", path.slice(2, 3))
print("slice(3, 4): ", path.slice(3, 4))
print("slice(4, 5): ", path.slice(4, 5))
print("slice(-1):   ", path.slice(-1))
print("slice(-2):   ", path.slice(-2))
print("slice(-3):   ", path.slice(-3))
print("slice(-4):   ", path.slice(-4))

Behavior on master:

slice(0, 1): Parent
slice(1, 2): Child
slice(2, 3): :prop
slice(3, 4): :prop:subprop     <---
slice(4, 5): :subprop          <---

slice(-1):   :prop:subprop     <---
slice(-2):   :prop:subprop
slice(-3):   Child:prop:subprop
slice(-4):   Parent/Child:prop:subprop

I believe the 3 marked results are not intended.
Behavior after this PR:

slice(0, 1): Parent
slice(1, 2): Child
slice(2, 3): :prop
slice(3, 4): :subprop
slice(4, 5): 

slice(-1):   :subprop
slice(-2):   :prop:subprop
slice(-3):   Child:prop:subprop
slice(-4):   Parent/Child:prop:subprop

I updated the unit tests to include 2 subnames, which reveals the problem.

---

Previously this test existed:

"Slicing on the length of the path should return the last entry."

But this is unintuitive, the last valid index in arrays is always size-1, not size. At least it's not clear to me why one would want this behavior, and it makes iterating through slices non-continuous. So I changed the test, but let me know if there was a reason for this.

[Mickeon]

This was added in #81822 so it may be nice to hear @nlupugla 's opinion as well.

[nlupugla]

Thanks for finding and fixing this! I haven't had a chance to carefully ponder your changes yet, but as far as the reasoning goes, my intention was for the nodepath slice method to have similar indexing behavior to arrays. So I agree with you that slicing on the last index should actually return empty and the last - 1 should return the last part of the path. Your output looks much more reasonable after your fix :)

[Ivorforce]

New behavior makes more sense to me.

[akien-mga]

Might be worth reviewing uses of slice() in the codebase to see if any expect the previous behavior (notably slice(-1) I guess).

Edit: Had a quick look, it doesn't seem to be used a lot, and the uses I found seemed ok.

[akien-mga]

Thanks!