feat: (responsive) defaults (e.g. width, height) when not explicitly provided

[manzt]

I think we can at least make the width and height optional so that users do not have to specify them when users use responsiveSize. But, yes, it is somewhat challenging to provide non-constant optimal width and height to individual tracks when the track/view structure becomes complicated.

Also, I was thinking that the appearance of some visual elements can be adaptive by default depending on the assigned size (e.g., hiding a color legend when the track height is too narrow). Another thing that we can figure out to make the responsiveSize more useful.

Originally posted by @sehilyi in https://github.com/gosling-lang/gos/issues/85#issuecomment-1021380417