Consider making `foo[[bar]]` and `foo[bar]` never expand when `bar` is "simple"

See https://github.com/posit-dev/air/issues/183

i.e. with

private$event_callback_counts[[domain]] <- private$event_callback_counts[[domain]] - 1

it should probably never break on the [[ and expand to

private$event_callback_counts[[domain]] <- private$event_callback_counts[[
    domain
  ]] - 
  1

instead it should probably give this, even though the first line exceeds the 80 char limit

private$event_callback_counts[[domain]] <- private$event_callback_counts[[domain]] - 
  1

Related to https://github.com/posit-dev/air/issues/163 because we would not want line breaks in $ to force line breaks here!

We will have to nail down what "simple" means, probably:

• Keywords like TRUE or NULL
• Identifiers
• Numerics / integers / complexes
• etc

Having a list of what makes something "simple" will likely be useful in other places too?

It should probably also respect persistent line breaks if you manually request the expansion? That's quite an edge case though.

Start by reading issues 183 and 163, then use the private$event_callback_counts[[domain]] example as the behavior to clarify. Define which expressions count as “simple,” decide how persistent line breaks interact with manual expansion, and verify that simple [[ and [ expressions avoid unwanted breaks without changing the intended formatting.