Antoine Stevan 1662e61ecb
deprecate def-env and export def-env (#10715)
follow-up to
- https://github.com/nushell/nushell/pull/10566

# Description
this PR deprecates the use of `def-env` and `export def-env`

these two core commands will be removed in 0.88

# User-Facing Changes
using `def-env` will give a warning
```nushell
> def-env foo [] { print "foo" }; foo
Error:   × Deprecated command
   ╭─[entry #1:1:1]
 1 │ def-env foo [] { print "foo" }; foo
   · ───┬───
   ·    ╰── `def-env` is deprecated and will be removed in 0.88.
   ╰────
  help: Use `def --env` instead


foo
```

# Tests + Formatting

# After Submitting
2023-10-19 13:50:16 +08:00
..
2023-02-27 13:58:56 -08:00