Implement support for mouse buttons 6 and above, as current browser support is lacking despite their existence and usability in other programs.
Building upon observations made in #6946: it seems that, despite mouse buttons 6 and above very much existing, and other programs being able to interface with them just fine, browser support just seems to not be there. The first three are universal, and the fourth and fifth are just about universally recognised for navigational purposes, but there's no good way for a browser-based program to hook into a sixth button or beyond on a mouse. From my testing, no browsers even register that such a button gets pressed, unlike other programs. The test case for the above linked issue (https://muzikbike.github.io/tests/mouse-button-recognition/index.html) uses MouseEvent.button to detect the first five mouse buttons. From what I've seen of this online, only five values from 0 to 4 are actually listed. I have no idea as to whether values 5 and above for this are supposed to be possible, or if it's restricted from 0 to 4 on purpose. Is this a cap that Ladybird can easily lift? Would there be comp