autosuggestion for memory variables in LG editor

[hibrenda]

Is your feature request related to a problem? Please describe.
When user is authoring in LG editor and typing dialog.xxx, they would like to get the auto suggested info for memory variables.

Describe the solution you'd like
Following scenarios need to be taken care.

• User defined
  eg. user.name; dialog.api_response, user.address.city
• prebuilt func
  eg. this.value, this.turnCount, turn.dialogevent.value
• luis related
  eg. #intent.score, @entity,turn.DialogEvent.Value.Entities.xxx