Support for section in tags

In typical elm apps, we have a large union type for global messages.
So we need grouping these messages using comments.
But elm-format doesn't seem to recognize this kind of commenting style.

What do you think about this?

## Union Type Declarations

Expected:

```elm
type Msg =
= Msg1
| Msg2

-- This comment introduces a section in the tags
| Msg3
| Msg4
````

Actual:

```elm
type Msg =
= Msg1
| Msg2

-- This comment introduces a section in the tags
| Msg3
| Msg4
````

*Edited: as @basile-henry pointed out, above example contains extra `=` signs. Just a mistake.*

Reproduce the Union Type Declarations example with elm-format and compare the Expected and Actual snippets. Trace how comments between union-type tags are handled, then verify that the section comment remains aligned as shown in Expected without changing other formatting.

Written by the indexing model from the issue text.