[FEATURE] Support BoxSelectTool-like range-setting for the RangeTool about bokeh

[bryevdv]

Please confirm this summary understanding:

Drag events initiated outside the existing range should define a new range to replace the old one (new behavior). Drag events inside the existing range or on its border should adjust the existing range (current behavior).

If so that seems like a reasonable flag to add. I don't have strong feelings about making it a new default cc @bokeh/dev

[droumis]

@bryevdv Yes, that's correct. Either as a flag or enabled by default is fine with me.

@mattpap Yes, it fits for CZI so I'll add the label. Given that this is really low priority, I'd like to take a first shot at this myself if that's ok.

[mattpap]

@droumis, should this be scheduled under CZI?