Add support for more slicing cases with SUBSTR

[knassre-bodo]

Currently, PyDough only allows string slicing in cases where the step is absent (or 1), and the start/stop are both non-negative (or absent), by translating to SUBSTR. However, SUBSTR has support for negatives, which we should also support. This means we can expand the conversion logic in convert_slice to handle the following additional cases:

1. s[-n:] -> SUBSTR(s, -n)
2. s[-n:-m] -> SUBSTR(s, -n, n-m))
3. s[:-n] -> SUBSTR(s, 1, LENGTH(s) -n)

If the dialect does not allow negative indices (like default ANSI), can change it so LENGTH(s) - n is always used for the second argument in the first 2 cases. This is viable because currently we require the slicing indices to be integer literals.

These expansions should all be tested, and should be noted in the function list documentation (which currently specifies the restricted semantics). The only restriction on slicing with s[...] syntax should now be that:

• The expression being sliced (s) must be a PyDough term (rather than a string literal)
• The step (e.g. c in s[a:b:c] must be 1 or must be absent
• The start/stop/step must all be litearls