fix date typing in arrow tables

[Fil]

uses the same duck type test as https://github.com/observablehq/inputs/pull/263

addresses https://github.com/observablehq/framework/issues/1376 (for the Plot part)

Note: it's not as good as full Arrow support (where the data would not get transformed into an array of Proxy objects, and valueof would use table.getChild, see #2030).