Support converting a Chart to a Vega editor URL

[jonmmease]

Closes #3188

This PR adds a Chart.to_url method that uses VlConvert to generate a Vega editor URL that opens the chart's specification in the online Vega editor.

import altair as alt
from vega_datasets import data

chart = alt.Chart(data.cars.url).mark_point().encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q',
    color='Origin:N'
)

print(chart.to_url())

https://vega.github.io/editor/#/url/vega-lite/N4Igxg9gdgZglgcxALlANzgUwO4tJKAFzigFcJSBnAdTgBNCALFAZgAY2AacaYsiygAlMiRoVYcAvpO50AhoTl4QpAE4AbFCDGEADpWQB6Q2DpQAdACtKdTOrhpV5qJkKGougLaG0mBHIBaeUVKV0oAATQARnMAJgBOczZDYLkTOVVKK0poEBkQTwyAa2VCAE9dTC1dCBJxfMwoSDoSJFQedQhVZXg7Oi0AeVVEEhBucsqtKAhPEjlNfIAPHqx1fuQQQS7QmuxMbvGKqo2AR1I5IjhFYl887jKVvq0AWTh1TEoAfUrVT4BxeadKBjEATY4gM4XYjXBxVaTcAAklDAjEwhS0On0Rh8fjk5gQV0YpAARuY4BBDMjUYUcf4AvZCJgfABWcxRVkxay5SRAA

cc @NickCrews

[binste]

Looks great @jonmmease. Not sure if that was the intention but it helps with #2875 which is nice.

Only added two minor comments.

[jonmmease]

Thanks for the review and suggestions @binste, I've accepted them.

Not sure if that was the intention but it helps with #2875 which is nice.

I hadn't come across that issue, but yeah this will provide a reasonable workaround for Visual Studio Code users.

[NickCrews]

Looks great! Thank you!

Could consider adding auto copy to clipboard support, but I lean towards that being out of scope

[NickCrews]

Consider: forcing all args to be keyword-only so that in the future, if we add more params to this function, they are not order dependent and we can re-order them as needed without breaking anyone. Not sure if this is overkill.

to_url(False) is also not very obvious what False does. Requiring a kwarg improves that.

On the other hand, requiring a kwarg is more typing.

I think I fall 60% in favor of requiring kwarg but am fine with keeping it as is

[jonmmease]

Good idea! Done in 5d440f9

[jonmmease]

Could consider adding auto copy to clipboard support, but I lean towards that being out of scope

Yeah, this is a possible future extension. Another idea is to optionally use the Python webbrowser module to open the URL in the editor.

[mattijn]

Thanks for the PR @jonmmease and reviews @NickCrews, @binste! This is great and valuable (especially in VSCode, like Stefan said.)