It works by automatically inserting a trailing comma before a line break in
multiline collection literals. Additionally, a colon is inserted between a key
and a value in dict literals.
Because of the language ambiguity regarding syntax of some incomplete collection
literals, colon is not inserted after the first key of a dict literal
(it's indistinguishable from a set literal with one item), and comma is not
inserted after the first item of a parenthesized tuple (it's indistinguishable
from a parenthesized expression).
The idea was taken from such implementation of Complete Current Statement for
JSON.
GitOrigin-RevId: 49ff9857d8c12476bfa9aacaed0b49faa99810fd
