Suggest network names when invoking code completion in networks

[rcjsuen]

Given this YAML file, we should suggest abc as it is a network could be set as a network for the test service. We should support this for both as an array item or as an object.

services:
  test:
    image: alpine
    networks:
      - 
networks:
  abc:

services:
  test:
    image: alpine
    networks:
      
networks:
  abc: