Trim completions and ensure final newline (#609)

Trim whitespace at beginning and end of generated completions.
Additionally, since some editors will automatically insert a final
newline into text files, make sure a final newline is present.

completions/just.elvish

@@ -1,4 +1,3 @@
-
 edit:completion:arg-completer[just] = [@words]{
     fn spaces [n]{
         repeat $n ' ' | joins ''

completions/just.powershell

@@ -1,4 +1,3 @@
-
 using namespace System.Management.Automation
 using namespace System.Management.Automation.Language
 

completions/just.zsh

@@ -60,4 +60,4 @@ _just_commands() {
     _describe -t commands 'just commands' commands "$@"
 }
 
-_just "$@"
+_just "$@"