Project: expression-editor
Task (challenging): Add autocompletion for the editor.

# Details

- Support auto complete builtin variables and operators.
  - builtin variables: foo, fooooo, bar, barrrr
  - operators: AND, OR
- Use prefix match to search.
- User can click on completion item to insert the completion item
  - Remove the common prefix already in the editor to avoid duplicate text after inserting
- Syntax highlighting still works after inserting completion

Here are some examples:
- When user input f, suggest `foo` and `fooooo`
- When user input fo, suggest `foo` and `fooooo`
- When user input foa, no suggestion
- When user input a, suggest AND
- When user input aN, suggest AND
- When user input aNb, no suggestion