Expand `forceMultiLineSpecifiers` to format imports of a single named import as a single line

[jakebailey]

This is a continuation of #473 (comment).

forceMultiLineSpecifiers forces imports to be vertical lists of single items, which is great for preventing merge conflicts. But, when importing one thing (which is surprisingly often), it looks like:

import {
    someFunction,
} from "some-library";

Which is maybe a little silly, especially when you may also be writing:

import someFunction from "some-library";

For libraries that have a default export instead.

It'd be nice if there were some way to say "a single import is okay", or similar. (per #473 (review))