this prevents the command being sent as plain text
this adds a `type` property to completions to decide which
parts should be inserted into the composer, hence deciding how
they will be rendered.
* suppress autocomplete when navigating through history
* only search for slashcommands if in the first block of the editor
* handle suffix returns from providers correctly
* fix SelectionRange typing in the providers
* fix bugs when pressing ctrl-a, typing and then tab to complete a replacement by collapsing selection to anchor when inserting a completion in the editor
* fix https://github.com/vector-im/riot-web/issues/4762