Add Nushell completion script (#1571)

completions/just.nu

@@ -0,0 +1,8 @@
+def "nu-complete just" [] {
+    (^just --dump --unstable --dump-format json | from json).recipes | transpose recipe data | flatten | where {|row| $row.private == false } | select recipe doc parameters | rename value description
+}
+
+# Just: A Command Runner
+export extern "just" [
+    ...recipe: string@"nu-complete just", # Recipe(s) to run, may be with argument(s)
+]