✨ First gh completions (#706)

Hi!
I could squeze some time to do the first `gh` completions.

On the future I will add the subcommands 👍🏼 

docs on 
https://cli.github.com/manual/

custom-completions/gh/gh-completions.nu

@@ -0,0 +1,17 @@
+# ------------------ nu-complete commands ------------------
+
+def "nu-complete gh" [] {
+    ^gh --help 
+    | lines 
+    | filter {|line| str starts-with "  " } 
+    | skip 1 
+    | parse "{value}: {description}" 
+    | str trim
+}
+
+
+# ------------------ export extern commands ------------------
+export extern "gh" [
+    command?: string@"nu-complete gh"
+    --help          # Show help for command
+]