Expose plotly.io.get_chrome()

[emilykl]

Closes #5272

Expose plotly.io.get_chrome() as a function which can be called from within a Python script.

get_chrome() accepts one optional argument, path, which is the location to download Chrome to; if not supplied, the default location defined by choreographer will be used.

This PR also slightly refactors get_chrome() into two functions:

• get_chrome(), which is intended to be called directly in code, and
• plotly_get_chrome(), which is a CLI wrapper for get_chrome() and intended to be called via the plotly_get_chrome command from the command line.

On this branch, running the following code should work:

import plotly.io as pio

pio.get_chrome()