Stunner - Enhance capabilitites for the FEEL editor (auto-completion)

This ticket is being incorporated by RHPAM-2022, which includes FEEL support for the given elements.

The goal for this ticket is to improve the editing capabilities for the FEEL editor, at least by adding auto-completion support (like the DMN requirement BAPL-1353).

Also consider capabilities can be also shared/reused from other editors, such as the script ones. So this ticket is also related with BAPL-678 in some way.

Reference: see following comments from dmarrazzo

In DMN context FEEL is considered to be a Friendly Enough Expression Language for decision experts.

BPMN designer are the same users of DMN and it's likely they have a good understanding of FEEL.

In order to provide a consistent experience to those users, it makes a lot of sense to express the gateway condition using FEEL. As FEEL auto suggestion will be available in DMN editor it could be shared with the BPMN editor, enabling a deeper control on the condition logic and a pleasurable experience to users.