add histograms on splom diagonals

Several other implementations of splom allow for a different plot type on the diagonal - which makes sense because the diagonal subplots are always just diagonal lines of points. What we really want is a simple view of the distribution of that one dimension in each diagonal subplot. For my current purposes the best view would be a histogram, and I suspect implementing only that would be a well-received improvement, but eventually box and violin could also be nice additions.

We anticipated this feature with a comment in #2555 and with a comment in the attributes for splom:

https://github.com/plotly/plotly.js/blob/f0fef6fe8c1c37786dcc9789bb60576073d2d0b7/src/traces/splom/attributes.js#L143-L158

That comment I think is right, trace.diagonal.type sets the trace type for all of the diagonal subplots, then we add more attributes into trace.diagonal that mirror the attributes available to histogram (and the other trace types if we add those). For histograms, trace.diagonal.marker.color should be inherited from trace.marker.color (and maybe similar with marker.line.width and marker.line.width?)

In addition, each dimension should be able to override everything about its own diagonal subplot, so trace.dimensions[i].diagonal should accept all the same options as trace.diagonal.

src/traces/splom/attributes.js の参照されているコメントと属性から始め、splom の対角サブプロットがどのように設定されているかを追ってください。trace.diagonal でヒストグラムのサポートを定義し、継承されたマーカー色と、trace.dimensions[i].diagonal による次元ごとのオーバーライドを含めます。対角サブプロットで、それらの設定パスが機能した状態でヒストグラムを表示できれば完了です。