Support `text-decoration` on Style object in Vaadin Charts 2.

[vaadin-bot]

Originally by basil.bourque

---

Vaadin Charts has a wonderful feature where the user can click on a legend item to remove or restore that series from the chart. Also nice is that a Style object can be set for when the series item is hidden.

Unfortunately, that Style object does not include the CSS property "text-decoration". I would like to have the text of the legend item appear with a strikethrough (https://en.wikipedia.org/wiki/Strikethrough) to more clearly indicate when a series is hidden.

---

Imported from https://dev.vaadin.com/ issue #15473