Charts: Styling improvements

Why: At the most basic level, it’s been 10 years: Our graphing libraries could go with a fresh coat of paint and some extra built-in styling functionality. How we’re winning: We have the best API. Our APIs in Python and Javascript are far and away the highest quality of any of these projects; we need the design and quality of the output to now exceed these other projects. How we’re losing: Other libraries like ChartJS look and feel better, and tools like Datawrapper allow you more flexibility at the graphing library level to build out charts that look great in publications. This is still difficult in Plotly.JS and we need to catch up to innovations of these other libraries.

- Support subtitles (https://github.com/plotly/plotly.js/issues/6856) - Define corner radius (https://github.com/plotly/plotly.js/issues/6853) - Support corner radius on paper (https://github.com/plotly/plotly.js/issues/6854) - Support border on paper (https://github.com/plotly/plotly.js/issues/6855) - Support shadow on paper (https://github.com/plotly/plotly.js/issues/6852) - Icons as markers (https://github.com/plotly/plotly.js/issues/6859) - Support color gradients (https://github.com/plotly/plotly.js/issues/6850) - Patterns instead of colors for fills (https://github.com/plotly/plotly.js/issues/6860) - Dotted grid (dots at cross points) (https://github.com/plotly/plotly.js/issues/6861) - Support online fonts (https://github.com/plotly/plotly.js/issues/6851) - Field for source text (https://github.com/plotly/plotly.js/issues/6857) - UI templates (https://github.com/plotly/plotly.js/issues/6858) - Weight and style for text (https://github.com/plotly/plotly.js/issues/4646) - Text weight (shadow) (https://github.com/plotly/plotly.js/issues/4646)