Extend Autocompletion with task and taskrun expressions and the root available variables

[brian-mulier-p]

As a follow-up to #3331 we can extend autocompletion.

In the order of priority

Prio 1 - DONE

Add autocompletion for root attributes when in an empty Pebble

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

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

Prio 2

• task.id
• task.type
• flow.tenantId
• envs DONE, not dynamically fetching envs for now
• globals DONE, note dynamically fetching globals for now
• for EE, namespace variables e.g. {{ namespace.myvar }}

Out of scope for now

Those can be explored later if time is available

Prio 3

• 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
• Add autocompletion for plugin defaults

Prio 4

Context aware:

• taskrun.parentId

• taskrun.value

• taskrun.iteration

• taskrun.id NEED TO BE CONTEXT AWARE, POSTPONE TO 0.18

• taskrun.startDate NEED TO BE CONTEXT AWARE, POSTPONE TO 0.18

• taskrun.attemptsCount NEED TO BE CONTEXT AWARE, POSTPONE TO 0.18

• parents NEED TO BE CONTEXT AWARE, POSTPONE TO 0.18