Move `:q<Enter>` tests to avi.command-line-mode-test
`:3` moves to line 3
Extract command-line-mode/line-number?
Extract command-line
Nest threading macros
Use cond to avoid multiple match problems
`:<Enter>` does nothing
`:badcommand<Enter>` doesn't change the cursor position
characters typed after `:` canbe deleted with backspace
Treat DEL as <BS> for now (because Mac)