Add Dask DataFrame plot test

[hoxbro]

Continue the work done in #1129 by adding simple plotting tests of dask.DataFrame and dask.Series.

I discovered a small bug that kde and density did error out because pd.melt was used.

Which means that something like this errors out on main:

import dask.dataframe as dd
import hvplot.pandas
import pandas as pd

y = ["A", "B", "C", "D"]
kind = "density"
df = dd.from_pandas(pd._testing.makeDataFrame(), npartitions=2)
df.hvplot(y=y, kind=kind)

[maximlt]

LGTM! I've added dask to the core test dependencies, its support should definitely be tested as often as possible.

It took me a while before I figured out this PR wasn't targeting main, once the branch it targets is merged I will point it to main so the commit history is as expected.

[hoxbro]

LGTM! I've added dask to the core test dependencies, its support should definitely be tested as often as possible.

Fine by me.

It took me a while before I figured out this PR wasn't targeting main, once the branch it targets is merged I will point it to main so the commit history is as expected.

Yeah, didn't want the merge conflict, so based in on the Polars PR. This should automatic be re-targeted when Polars Pr are merged. Though, something like this could be needed git merge main --strategy-option ours.