support using replaces-step with more than one step

[michaelficarra]

Currently, replaces-step is used only when replacing a single step with a single replacement step (it is what gets the step numbering of the replacement correct). We can support replacing with more than one step by treating the replacement steps as if they are nested a level deeper. So, for example, when replacing step 4 with 2 steps, we would number them 4.a and 4.b.

Current workaround is to use a single step that says "perform the following steps" with some sub-steps. That's not great.

[michaelficarra]

Turns out in ecma262 we were also sometimes replacing with more than one step, which on at least one occasion ran into the numbering oddity. I've opened https://github.com/tc39/ecma262/pull/2688 to use this workaround in the meantime.