Add a config for specifying min bar size

[kanitw]

In https://github.com/vega/vega-lite/pull/8349, I fix #8350 by making minimum bar band = 0.25. (See this line) Ideally we should make this configurable. The question is where to add this config.

Note that this config is only applicable to bar/rect mark. So extending RectConfig with a new property could make sense.

Maybe config.bar/rect.minBandSize given we already have continuousBandSize and discreteBandSize?

cc: @domoritz