Give string expansion an (optional) parent pgroup. This is threaded all
the way into eval(). This ensures that in a mixed pipeline like:
cmd | begin ; something (cmd2) ; end
that cmd2 and cmd have the same pgroup.
Add a test to ensure that command substitutions inherit pgroups
properly.
Fixes#6624
This commit recognizes an existing pattern: many operations need some
combination of a set of variables, a way to detect cancellation, and
sometimes a parser. For example, tab completion needs a parser to execute
custom completions, the variable set, should cancel on SIGINT. Background
autosuggestions don't need a parser, but they do need the variables and
should cancel if the user types something new. Etc.
This introduces a new triple operation_context_t that wraps these concepts
up. This simplifies many method signatures and argument passing.