feat: add string permutations one-liner to python_oneliners.md

@@ -209,3 +209,10 @@ import math; fact = math.factorial(n)

[s[i:j] for i in range(len(s)) for j in range(i + 1, len(s) + 1)]

```

**Example:** `"abc" → ["a", "ab", "abc", "b", "bc", "c"]`

## Find all permutations of a string

```python

from itertools import permutations; ["".join(p) for p in permutations(s)]

```

**Example:** `"abc" → ['abc', 'acb', 'bac', 'bca', 'cab', 'cba']`