sub() STDLIB function: formalize the syntax for using capturing groups in the replace string

[a-frantz]

The SPEC (https://github.com/openwdl/wdl/blob/wdl-1.2/SPEC.md#sub) is explicit that the pattern string must be POSIX ERE compliant, but doesn't say anything about what's permitted in the replace string. This leaves it up to the engine whether capturing groups from the RE can be used in the replace string, and there isn't any uniformity as to the syntax for that, leading to non-portable WDL.

e.g. sprocket uses the regex crate which supports $<ref> syntax for capturing groups in the replace string. I think that miniwdl uses \<ref> syntax (or that's the syntax for the Python sub() func). No idea what cromwell does.

One standard should be enforced instead of leaving it up to the engine. I don't think the specific standard ($ref or \ref) matters, so long as it's portable across engines.