**Summary** Fix the formatter crash with `x[(1) :: ]` and related code.
**Problem** For assigning comments in slices in subscripts, we need to
find the positions of the colons to assign comments before and after the
colon to the respective lower/upper/step node (or dangling in that
section). Formatting `x[(1) :: ]` was broken because we were looking for
a `:` after the `1` but didn't consider that there could be a `)`
outside the range of the lower node, which contains just the `1` and no
optional parentheses.
**Solution** Use the simple tokenizer directly and skip all closing
parentheses.
**Test Plan** I added regression tests.
Closes#5733
a51606a10a