Implement new lambda syntax

[gamebox]

The new lambda syntax - per this Zulip thread - is the below:

|a, b| expr

|a, b|
    a_var = Something
    expr

This should be supported alongside the existing syntax for the time being. Unlike previous changes, I'd like to say that the formatter should just formats all lambdas to the new syntax instead of using a MigrationFlag.

[Anton-4]

We've got PRs with breaking changes going that affect many files, I would delay getting started on this @gamebox to avoid thousands of merge conflicts.