The new query system already supports arbitrary Region queries, but some tutorial notebooks search by an (ra, dec) point and it would be nice if that was supported directly instead of needing to convert to a `Region` manually.

Add support for this in three places:

• POINT(ra, dec) syntax in where strings (the syntax is already in the parser, it’s just not hooked up to anything)

• binding a variable in a where string

• ExpressionFactory

For the ExpressionFactory and bind cases, it should be possible to use either an astropy.coordinates.SkyCoord or lsst.sphgeom.LonLat