Fluid mode's automatic detection is nice, but there can be a layout jump once the video dimensions are known. We have the vjs-16-9 and vjs-9-16 classes already, but square and portrait are also common now.
Follow-up for #5471
This makes the breakpoints option and `breakpoints()` method clearer and introduces the responsive option and `responsive()` method, which will turn on the breakpoints.
The return value of `currentBreakpoint()` was simplified to only ever return a string (empty if none).
Also, added convenience methods: `responsive()`, `getBreakpointClass()`, and `currentBreakpointClass()`.
