Introduces `_regexp-step`, a step command which additionally allows for
stepping into a target function. This change updates `step` and `s` to
be aliases for `_regexp-step`.
The existing `sif` alias ("Step Into Function") is not well known
amongst users. This change updates `step` and `s` to also work like
`sif`, taking an optional function name.
This is implemented to not break uses of `step` or `s` with a flag, for
example running `step -r func_to_avoid` works as expected.
6e3c7b8244