pragma: optional by default

[mattfysh]

build a way for queries to switch to 'optional by default' mode, which acts the same as adding ? to all assignment statements & object literal entries

in this new mode, add support for ! to indicate required values

a couple of options:

1. add triple-dash comment to top of query, e.g. --- optional
2. context chain modifier, e.g.

set my_var = @optional -> {
  all,
  entries,
  nullable,
  except!: #this .selector
}