Implement simple autocomplete in editor

[mint-thompson]

Transferred from internal JIRA issue originally created 2020-06-16.

Implement a simple autocomplete in the editor. This autocomplete would work similarly to how the basic VS Code extension autocomplete and will give suggestions for words that have already been typed in the editor. This won't do any sort of checking against what is valid in FHIR based on the context of what you have typed.