Allow manual selection of panningMode

When panning on a plot, the pan mode is currently hardcoded depending on the initial movement of the pan in [PanGeomFeedback.kt](https://github.com/JetBrains/lets-plot/blob/master/plot-base/src/commonMain/kotlin/org/jetbrains/letsPlot/core/interact/feedback/PanGeomFeedback.kt).

If the initial pan movement is horizontal or vertical, it sets the `panningMode` to `PanningMode.HORIZONTAL` or `PanningMode.VERTICAL`.

Can we expose an option to manually set the `panningMode` to always be `PanningMode.FREE`?

Start in plot-base/src/commonMain/kotlin/org/jetbrains/letsPlot/core/interact/feedback/PanGeomFeedback.kt, where panningMode is selected from the initial pan movement. Trace how pan options are exposed and determine where a manual PanningMode option belongs. Done means callers can select PanningMode.FREE instead of relying on the initial movement.

Written by the indexing model from the issue text.