feat: Support datetime.(date|datetime) in Expression(s)

[dangotbanned]

Closes #3652

Example

import datetime as dt

import altair as alt
from vega_datasets import data

base = (
    alt.Chart(data.stocks.url)
    .mark_line()
    .encode(x="date:T", y="price:Q", color="symbol:N")
)

predicate = alt.datum["date"] <= dt.date(2005, 1, 1)

chart = (
    base.transform_filter(predicate) | base.transform_filter(~predicate)
).resolve_scale(y="shared")
chart

[jonmmease]

This looks great @dangotbanned, I really like the emphasis on improving the expression language API

[dangotbanned]

This looks great @dangotbanned, I really like the emphasis on improving the expression language API

Thanks @jonmmease, appreciate the review