Summary

Fixes #1220. The base case issue here was that dangling comments of mapped types within a union type were getting printed twice, but another portion was that they were positioned incorrectly in the first place. Consider:

| {
  // this dangling comment gets duplicated
  [A in B]: T
}// this dangling comment gets duplicated;

The comment is printed twice, so let's fix that first. The root cause was that TsUnionType formats all of the comments of each variant directly. All leading, dangling, and trailing. For most nodes this isn't a problem since they don't do any special handling for them. But for mapped types, the dangling comments are directly formatted by the node itself (and moved to act as leading coments instead), so when the union then formatted the comments as dangling comments, they were printed twice. The same was actually also true for empty object-like and tuple types, which also format their own dangling comments.

The solution here was just to opt-out of re-printing the comments in the union for those types of nodes. While working in there, I also copied over the rest of prettier's logic for trailing comments in mapped types as well.

This PR also addresses the break-mode tests from Prettier, where the mapped type should only break over multiple lines if a newline occurs between the { and the start of the property name. Newlines in any other position don't cause the type to expand unless the inner groups break.

Test Plan

The Prettier snapshots cover both of these cases, break mode for itself, and 11098 for the comment positioning.