Refmting multi-line ppx strings

Actual:

let compose:
  _ => unit

    [@@ocaml.deprecated
      {|
Use the |> as an infix operator to chain the
result of one function into another:

`compose(f, g, h)(x)`
in JS goes to
`x |> h |> g |> f`
in Reason.
|}
    ];

Expected:

let compose: _ => unit [@@ocaml.deprecated {|
Use the |> as an infix operator to chain the
result of one function into another:

`compose(f, g, h)(x)`
in JS goes to
`x |> h |> g |> f`
in Reason.
|}];

Or if the first line was really long:

let compose:
  oneThing =>
  anotherThing =>
  oneMore =>
  unit
  [@@ocaml.deprecated {|
Use the |> as an infix operator to chain the
result of one function into another:

`compose(f, g, h)(x)`
in JS goes to
`x |> h |> g |> f`
in Reason.
|}];

Start by reproducing the formatting examples in the issue and locating the formatter entry point that handles PPX attributes and multi-line strings. Done means the formatter keeps the attribute on the appropriate line while preserving the string contents and produces the expected output for both examples.