abstract out shell script creation

Shell scripts are generated in quite a fragile way right now.

Real gitlab runner has a more general set of classes that generate steps for each kind of shell. We should do similar to make it easier to support other shells (such as the now default pwsh which we do not support yet)

for each of sh, bash, cmd, powershell
echo commands before running
escape strings for above echo

Edited May 15, 2022 by Ian Norton

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