add demo showing value and line in axis overlay

[RonnieLincoln]

[leeoniya]

@RonnieLincoln thanks so much! it's clear you did your homework, especially with using cursor.bind :)

i refactored things a bit and pulled it out into a plugin, and extracted some of the static styles to CSS. i understand why you made some of the choices that you made, and some of the things i changed (like not reading back from uPlot instance) may not be to everyone's liking, and in some ways makes the setup a bit more fragile, but feel free to incorporate any of the refactorings that you like into your own code.

https://leeoniya.github.io/uPlot/demos/axis-indicators.html

see you in 2025!