Add default values to prompts

[lnikell]

Add an option to set default values to the prompts.
Just add an extra field below the variable name input

Default values for prompts will be used two ways:

• When we execute the template from VS Code Extension we need to add to the message also information about default value and in case user just press enter we will use it.
  
• For marketplace public templates animations. Example. Instead of variable-one, variable-two we will be able to use default values, which will make that animation more real.