sh.lang: Complete variable definition command

While working on a fix for #40 (closed), I realized that variable definition command needed a complement. Sorry for not having seen this before, when !86 (merged) was not yet merged.

Adding: "stand-alone-variable-definition" could be improved on the model of "variable-definition-command", by the use of a container context, because it is also an instruction that end at line-end or by a boundary operator. But I think it's enough for this MR.