Add support for newlines in identity()

[julienfalque]

This PR allows writing complex <identity()> expressions in a more readable way, e.g.:

Foo:
  foo:
    __construct: <(new Some(new Very(new Complex(new Expression(), new With(), new Many(), new Arguments())))>

can become

Foo:
  foo:
    __construct: |
      <(new Some(
        new Very(
          new Complex(
            new Expression(),
          ),
          new With(),
          new Many(),
          new Arguments(),
        ),
      ))>

I focused on <identity()>. Are there other kind of values I overlooked that would conflict with this? Or that could benefit from newlines support as well?

[theofidry]

Are there other kind of values I overlooked that would conflict with this? Or that could benefit from newlines support as well?

No idea :) The PR looks good though. Is there anything else or can that be merged?

[julienfalque]

I'm not very confident this won't break things, maybe worth try it in some projects? Otherwise this can be merged IMO.

[theofidry]

Sorry for the delay, thank you @julienfalque :)