feat(env): complete commands and variable assignments

[scop]

Refs #1111

[scop]

Test coverage is likely something to expand before we merge this.

Would be nice to also address --*-signal=SIG too, but that's not a blocker I think.

[akinomyoga]

I think we can add tests.

[akinomyoga]

I added further commits because -* may also be treated as the command name after any variable assignments *=*. For example, in the case of env foo=bar --debug s, --debug is actually treated as the command name by env. Commit 71b7fb2 handles it.

However, I noticed that if we pass the position of --debug to _comp_command_offset, _comp_load produces error messages because it internally checks the command name by type -P "$cmd". So I first fixed _comp_load (and _comp_realcommand, which uses type -P in a similar way) in commit 883946d.

In addition, after the option -- and -, any non-variable-assignment word (including -*) is treated as the command name. Commit 0cd2883 implements the handling of these.

[scop]

Nice! Good to go as far as I'm concerned (GH won't let me approve this because I'm the original submitter).

[akinomyoga]

Thanks!

There are actually similar cases for type -P, but I think we should consider them in another PR.