Add formatting for extension type declarations.

[lrhn]

Attempt to format extension types. Since they are experimental, the syntax and formatting can still change before launch,
but this should make it possible to format code containing extension type declarations.

Which will make writing tests much nicer.

[munificent]

Couple of changes then LGTM.

[munificent]

This doesn't look great to me. There's an implicit principle in the formatter that adjacent lines shouldn't have the same indentation but for different reasons. In this case, line three is implemented +4 because of the representation type's parameter list and the next line is indented +4 because it's a split implements clause.

If the representation type splits, I would indent it past the implements clause, like:

@Anno<int, int>()
extension type const A<S, T>.name(
        @required Map<int, int> a)
    implements I<S>, J<T> {}

(Eventually, with the new style, it will look more like, maybe:

@Anno<int, int>()
extension type const A<S, T>.name(
  @required Map<int, int> a,
) implements I<S>, J<T> {}

)

To implement this, try moving:

builder.startRule(CombinatorRule());

Above:

    visit(node.typeParameters);
    visit(node.representation);

And then you might need to wrap another nestExpression() unnest() pair around:

    builder.startRule(CombinatorRule());
    visit(node.implementsClause);
    builder.endRule();

(I could be wrong about this. There tends to be some trial and error with hacking on the formatter. That's one of the reasons I want to move to a better internal representation.)

[lrhn]

We want the "primary construtor parameter" to be indented more iff there is a following implements clause, and they are all on separate lines.

I seem to have succeeded in that using:

    builder.nestExpression();
    visit(node.typeParameters);
    visit(node.representation);
    builder.unnest();

    builder.startRule(CombinatorRule());
    visit(node.implementsClause);
    builder.endRule();

[lrhn]

I believe the indentation formatting has been fixed (otherwise feel free to tinker more), so landing!