Bug 260229

The current implementation of text-wrap: balance does not account for ::first-line styling, text-indent, and white-space-collapse.

Indentation and ::first-line styling are both CSS factors related to the first line of inline content (or, at least in the case of indentation, the first line after a forced line break). There is also the question of whether leading/trailing white space with `white-space-collapse: preserve` should count as content to be balanced. It turns out that we should always trim leading/trailing preserved white space, except if it is leading preserved white space on the first line (or the first line after a forced line break).