Pipeline parsing of "pipeline.yml" failed (Expected identifier to start with a letter, got ?)

Similar to #584 I'm getting the error in the title trying to compare `$?` in a pipeline command step. Neither of these work regardless of quoting:

```
if [[ "\$?" == "0" ]]; then ...
if [[ "$$?" == "0" ]]; then ...
```

Start by reading the related issue #584 and locating the pipeline parser path for command-step expressions. Reproduce the failure with the two snippets from this issue; done means comparing `$?` with `0` in a pipeline command step no longer produces the identifier parsing error.

Written by the indexing model from the issue text.