Make slicing behavior consistent for highly negative offsets

[orlp]

This is the current behavior of pl.Series(["abcd"]).str.slice(i, 2), and the other (non-string) slice operations behave similarly:

       -7 ab 
       -6 ab 
       -5 ab 
0 ab   -4 ab 
1 bc   -3 bc 
2 cd   -2 cd 
3 d    -1 d  
4
5

This inconsistency between the left-hand-side and right-hand-side should be fixed, and instead we'd have the following behavior:

        -7    
        -6    
        -5 a  
0 ab    -4 ab 
1 bc    -3 bc 
2 cd    -2 cd 
3 d     -1 d  
4 
5 

To translate from Polars-style offset, length to Python's start_index, end_index the following logic should apply consistently everywhere:

# Signed arithmetic (allows negatives).
start_idx = len(s) + offset if offset < 0 else offset
stop_idx = start_idx + length

# Clamp negatives / out-of-bounds.
def clamp(x, lo, hi): return max(lo, min(x, hi))
start_idx = clamp(start_idx, 0, len(s))
stop_idx = clamp(stop_idx, 0, len(s))