* Add `editor::RewrapOptions::line_length` to, optionally, override the
  line length used when rewrapping text.
* Update `editor::Editor::rewrap_impl` to prefer
  `editor::RewrapOptions::line_length`, when set.
* Add a `line_length` field to the `vim::rewrap::Rewrap` action.
* Update the `:reflow` vim command with `vim::command::VimCommand::args`
  so as to be able to parse the provided argument as `usize`, ensuring
  that no effect is taken if the argument can't be parsed as such.

Release Notes:

- N/A