Add completions for nu (#5700)

docs/sample_config/default_config.nu

@@ -121,6 +121,22 @@ module completions {
     --verbose(-v)                                   # be more verbose
     --help                                          # Display this help message
   ]
+  # Completions for nu itself
+  export extern "nu" [
+    --help(-h)                # Display this help message
+    --stdin                   # redirect the stdin
+    --login(-l)               # start as a login shell
+    --interactive(-i)         # start as an interactive shell
+    --version(-v)             # print the version
+    --perf(-p)                # start and print performance metrics during startup
+    --testbin:string          # run internal test binary
+    --commands(-c):string     # run the given commands and then exit
+    --config:string           # start with an alternate config file
+    --env-config:string       # start with an alternate environment config file
+    --log-level:string        # log level for performance logs
+    --threads:int             # threads to use for parallel commands
+    --table-mode(-m):string   # the table mode to use. rounded is default.
+]
 }
 
 # Get just the extern definitions without the custom completion commands