Add Inputs, Outputs, Trigger and Variables autocompletion

[anna-geller]

Feature description

Providing autocompletion after the following components would significantly improve the UX of writing flows and help onboard new users to the product:

• {{ inputs.[list available inputs user can select from] }}
• {{ outputs.[list available tasks to select from] }}
• {{ outputs.mytask.[list available outputs of that task] }}
• {{ vars.[list available variables] }}
• {{ trigger.[list available trigger variables] }}