Add rectRounded point style

[cmoel]

Hello! This adds a new point style, rectRounded, and updates the docs that mention the valid values of pointStyle.

[etimberg]

Changes look good. Can you update https://github.com/chartjs/Chart.js/blob/master/samples/line/point-styles.html as well?

[simonbrunel]

Shouldn't be roundedRect instead?

[cmoel]

@etimberg Thanks! Will do.

@simonbrunel I originally called it roundedRect but decided to go with rectRounded to stay consistent with rectRot and crossRot (I should have put that in the description... sorry!). That said, I'm happy to change it to roundedRect if that's what you all prefer.

[simonbrunel]

@cmoel I like consistency, so would certainly have the same approach as you :) I personnaly prefer roundedRect (and roundedRectRot), which seems more standard. @chartjs/maintainers your thoughts?

[etimberg]

I do agree that roundedRect sounds the best, but rectRounded does match the other options. I don't have a strong preference either way

[btahir]

I am trying to understand the thinking behind this...why would you not give an option to adjust the border radius of a rounded rectangle? It seems it's a hard coded thing and the only way around it is to do a custom canvas object or image?