Fix a bug with `:norm` that disallowed trailing whitespace.

Commands that accept generic args (defined with `args()`) now preserve
trailing whitespace, while commands that accept filenames (defined with
`filename()`) have whitespace pre-trimmed. This allows, for example, 
`:norm I  ` to correctly insert spaces, matching NeoVim's behavior.

Release Notes:

- vim: Fixed `:norm` command to preserve trailing whitespace in
arguments (e.g., `:norm I ` now correctly inserts two spaces)

---------

Co-authored-by: dino <dinojoaocosta@gmail.com>