Rewrite `.list$foo` as `.list[["foo"]]`

[gadenbuie]

Another idea for the roadmap: I often start writing code like this

.list$foo$bar$baz

where auto-complete helps fill in the list items. But later I'll refactor to ensure that I'm using [[ to access list items safely.

.list[["foo"]][["bar"]][["baz"]]

An addin to toggle between these formats would be an awesome addition.