Detect whitespace-only changes and call them out explicitly

[JakeWharton]

Usually these render the same visually but are otherwise impossible to detect.

For example, the CLDR updated in JDK 20 to use narrow non-breaking spaces instead of plain spaces in-between the time and "AM"/"PM". This is impossible to detect visually when in a monospaced output context.

Other examples would be the use of regular non-breaking spaces, zero-width spaces, tabs, etc.

[evant]

As mentioned in https://github.com/willowtreeapps/assertk/issues/509#issuecomment-1847692756, tabs are rendered, but not other whitespace chars currently