Link the short form syntax of depends_on to the defined service for textDocument/documentHighlight requests

[rcjsuen]

We should highlight the two instances of db and redis correctly if we receive a textDocument/documentHighlight request. We should also return a single element if the cursor is on web.

services:
  web:
    build: .
    depends_on:
      - db
      - redis
  redis:
    image: redis
  db:
    image: postgres