Add auto-completion as a general properties panel concern

[nikku]

Is your feature request related to a problem? Please describe

Auto-completion / hinting is a feature developers generally expect to be there in modern IDEs / text editors. For the properties panel we do not have it. However, thinking in extension points it would provide our users with a powerful extension point to hook into. Some examples:

• Auto-complete variables
• Auto-complete names (i.e. well known names of business objects / process participants)
• Auto-complete form keys or refs

Describe the solution you'd like

Provide a generic auto completion infrastructure for text-based inputs

• [ ] Can be implemented easily as an auto-completion provider
• [ ] Gets enough context to sensible completion can take place
• [ ] Completion is context aware and completes words or the whole selection

Describe alternatives you've considered

Keep things as is and resort to custom auto-completing text boxes.

Additional context

Related to https://github.com/bpmn-io/internal-docs/issues/360

[nikku]

CC @barmac Likely direction to go if we support completion of linked resources.