just-completion: "export def" -> "export extern" (#860)

Changing "export def" to "export extern" to not shadow the original
command as it would disallow anything not defined in the re-definition

There seem to be similar cases in completions for (naive search with "rg
'export def'")
- mask
- winget (exports some defs and some externs. So i dont know if those
defs are actually additional functionality)
- pnpm (only exports some 'nu-complete' defs)
- pass (only exports some 'nu-complete' defs)

I don't feel comfortable changing those as i am not familiar with them.

custom-completions/mask/mask-completions.nu

@@ -44,13 +44,7 @@ def "nu-complete mask args" [context: string, offset: int] {
     $rt
 }
 
-export def main [
+export extern "mask" [
     recipes?: string@"nu-complete mask recipes"
     ...args: any@"nu-complete mask args"
-] {
-    if ($recipes | is-empty) {
-        ^mask --help
-    } else {
-        ^mask $recipes $args
-    }
-}
+]