Implement auto-completion

[Skaiir]

Is your feature request related to a problem? Please describe.

The feel editor supports auto-completion, but feelers does not.

Describe the solution you'd like

We need to think about two scenarios: context-sensitive and context-agnostic auto-completion, feel variable completion would belong to the former, and FEEL functions to the later

I'm not sure we really need auto-completion for the feelers-specific language tags, but for the feel expressions, we certainly would like to bring them in.

I would say first to focus on the context-agnostic part of the equation, and then think about how to do context tracking at a later time.

Describe alternatives you've considered

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