Autocompletion - Propose user with Pebble autocompletion and the root available variables

[brian-mulier-p]

Feature description

As a follow-up to #3331 we can add some autocompletion when user is in pebble or just typed ":", but also for pebble functions & filters

• Add autocompletion for every root attribute when in empty Pebble
• Add autocompletion for Pebble functions when we're in Pebble {{...}} and there's a word ahead of the cursor
• Add autocompletion for Pebble filters when we type "|" inside Pebble

you type {{ and then cmd+Space will give you:

• inputs
• outputs
• vars
• trigger
• execution
• taskrun
• task