sh.lang: Introduce "generic-command" context
There are some other things to improve or fix, for sure, but I think I can now give a sort of general response to #40 (closed), with this "generic-command" context (including in particular echo command).
It is essentially a copy of "variable-definition-command" context (without the variable-specific part), so most of the work to properly introduce it has already been done in previous MR.