feat: supplement list of typical Package contents

- .sublime-commands after menus and before `metadata`
  since commands seem relatively common in Packages.

- .sublime-completions before `snippets` since, according to the
  official documentation, a Snippet is just a Completion with more
  flexible (and more readable) content formatting.

Author: vwheeler63

docs/guide/extensibility/packages.md

@@ -72,10 +72,12 @@ Typical resources found in packages include:
 - key maps (`.sublime-keymap`)
 - macros (`.sublime-macro`)
 - menus (`.sublime-menu`)
+- commands (`.sublime-commands`)
 - metadata (`.tmPreferences`)
 - mouse maps (`.sublime-mousemap`)
 - plugins (`.py`)
 - settings (`.sublime-settings`)
+- completions (`.sublime-completions`)
 - snippets (`.sublime-snippet`)
 - syntax definitions (`.sublime-syntax`, `.tmLanguage`)
 - themes (`.sublime-theme`)