ScriptDocValues.Longs exposes a .date member that other scripting engines use to get convenient access to date components.

For example doc['field'].date.monthOfYear.

Expressions does this inconsistently, it supports a few (but not all) of these methods, but via a different syntax without the .date, and sometimes with different semantics (!) because it uses java.util.Calendar.

For expressions this pr adds .date working the same way/syntax is it does for other engines, and undocuments the old methods: the old methods are left unchanged for anything using them. For painless it adds ReadableDateTime to the whitelist so the same syntax works there too.

We can also fix a few other inconsistencies in expressions, like supporting .length/.size() for the count of values for the field to match other engines: this way its more of a proper subset for most uses.