merge venv_deactivate.nu with venv, move venv from venv/

make similar to `conda.nu`
- add `export` keyword

modules/virtual_environments/venv/venv.nu → modules/virtual_environments/venv.nu

@@ -1,4 +1,4 @@
-def --env venv [venv_dir] {
+export def --env activate [venv_dir] {
  let venv_abs_dir = ($venv_dir | path expand)
  let venv_name = ($venv_abs_dir | path basename)
  let old_path = $env.PATH
@@ -16,3 +16,10 @@ def "venv-path" [venv_dir] {
  PATH: ($env.PATH | prepend $env_path)
  }
 }
+
+export def --env deactivate [] {
+ $env.PATH = $env.VENV_OLD_PATH
+ hide-env VIRTUAL_ENV
+ hide-env VENV_OLD_PATH
+
+}