improve jsdoc conversion

[Rich-Harris]

this does two things — it ensures that we don't insert double newlines...

...and ensures that we do add a newline if either the current node or the preceding node spans multiple lines. In other words we can convert this

/** @type {Foo[]} */
let foo = [];

/** @type {Bar[]} */
let bar = [];

to this

let foo: Foo[] = [];
let bar: Bar[] = [];

but if either the foo or bar declaration spans multiple lines, we want to have a newline between them after the conversion.