allow configuring smart tab to always trigger completion instead

[doy]

it's always been pretty difficult to reliably use tab to select completions - if autocompletion is enabled, it's very easy to press tab before the autocomplete menu loads, which previously just inserted a literal tab and now jumps the cursor around unexpectedly. i'd really like to be able to use tab solely for completion, and this accomplishes that pretty well. the main thing i'm unsure about here is how the configuration should work here - it feels like this would be a lot simpler if i could just map the smart tab action to an arbitrary command, but MappableCommand is defined in helix-term, so it's not available to use as part of the editor.smart-tab config section which is defined in helix-view. that said, there aren't really that many other actions that really make sense to be triggered by smart tab, so maybe this is fine? we could also potentially replace enable: true with mode: InsertTab or something along those lines, but i don't really have strong feelings either way there.

[pascalkuthe]

smart tab needs larger changes (particularly to integrate with snippets).

Instead of changing the existing config we should allow people to make arbitrary keybindings mapped to smart tab. That is blocked on some larger architectural changes.

I am also more and more convinced that tab is overused. I actually want to remove tab from the default completion menu bindings because it conflicts with snippets (its annoying you have to press enter before you can tab trough the snippet). <c-n> and <c-p> are better bindings.

So I would rather not go forward with this instead wait on the larger refactors that make smart tab bindable to anything. You can keep this as a personal branch but I don't think I would merge this.