Closes#9003.
This PR changes `group-by` so that its optional argument is interpreted
as a cell path. In turn, this lets users use `?` to ignore rows that are
missing the column they wish to group on. For example:
```
> [{foo: 123}, {foo: 234}, {bar: 345}] | group-by foo
Error: nu:🐚:column_not_found
× Cannot find column
╭─[entry #3:1:1]
1 │ [{foo: 123}, {foo: 234}, {bar: 345}] | group-by foo
· ─────┬──── ─┬─
· │ ╰── cannot find column 'foo'
· ╰── value originates here
╰────
> [{foo: 123}, {foo: 234}, {bar: 345}] | group-by foo?
╭─────┬───────────────╮
│ 123 │ [table 1 row] │
│ 234 │ [table 1 row] │
╰─────┴───────────────╯
```
~~This removes the ability to pass `group-by` a closure or block (I
wasn't able to figure out how to make the 2 features coexist), and so it
is a breaking change. I think this is OK; I didn't even know `group-by`
could accept a closure or block because there was no example for that
functionality.~~
9ce61dc677