fix: merge simple property completion

[p-spacek]

What does this PR do?

try to merge simple property completion from anyOf objects.
It's used syntax for choice in snippets: ${1|one,two,three|} https://code.visualstudio.com/docs/editor/userdefinedsnippets#_choice

original:

Screen.Recording.2022-03-22.at.13.37.14.mov

after this PR:

Screen.Recording.2022-03-22.at.13.13.26.mov

schema

{
  "anyOf": [
    {
      "properties": {
        "prop": {
          "const": "const value"
        }
      }
    },
    {
      "properties": {
        "prop": {
          "const": "const value2"
        }
      }
    },
    {
      "properties": {
        "prop": {
          "type": "boolean",
          "default": false
        }
      }
    },
    {
      "properties": {
        "prop": {
          "type": "null"
        }
      }
    }
  ]
}

What issues does this PR fix or reference?

no ref

Is it tested? How?

modified existing and add new

[coveralls]

Coverage increased (+0.3%) to 81.849% when pulling e7205fd on p-spacek:fix/merge-simple-property-completion into 01d4e9e on redhat-developer:main.