Deprecate .ref() instead of removing it

[mattijn]

Fix #3059.

This PR reintroduce the .ref() function which was removed in Altair 5.0.0.

While the function is not needed anymore it is better to deprecate first to ease transition.

Now the following from the Altair Tutorial on selections:

import altair as alt
from vega_datasets import data

weather = data.seattle_weather()

interval = alt.selection_interval(encodings=['x'])

base = alt.Chart(weather).mark_rule(size=2).encode(
    x='date:T',
    y='temp_min:Q',
    y2='temp_max:Q',
    color='weather:N'
)

chart = base.encode(
    x=alt.X('date:T', scale=alt.Scale(domain=interval.ref()))
).properties(
    width=800,
    height=300
)

view = base.add_selection(
    interval
).properties(
    width=800,
    height=50,
)

chart & view

d:\Software\mattijn\altair\altair\utils\deprecation.py:65: AltairDeprecationWarning: 'ref' is deprecated. No need to call '.ref()' anymore.
  warnings.warn(message, AltairDeprecationWarning, stacklevel=1)
d:\Software\mattijn\altair\altair\utils\deprecation.py:65: AltairDeprecationWarning: 'add_selection' is deprecated. Use 'add_params' instead.
  warnings.warn(message, AltairDeprecationWarning, stacklevel=1)

And a working plot.

[binste]

Looks good to me! Only one minor comment.