Update how we print types in error messages

[cpovirk]

jspecify/jspecify#129 proposes some differences from what we do now, including:

• using a ! suffix for non-nullable types
  • including usages of E inside @NullMarked class Foo<E> { ... }
• outputting Foo?<Bar> instead of Foo<Bar>?
• maybe other stuff?

(There are also larger questions about whether to use shorthand at all in error messages or, if we do, whether to supplement it with @Nullable Foo-style information.)

I don't know if we're hoping to make a decision on that issue in the 0.3 timeframe, so I'm leaving this issue without a milestone for now.