[Feature Request] Add `HasDimension` to the public API

[wojciech-graj]

What is the feature ?

The HasDimension trait is required when implementing IntoTextStyle, as into_text_style is a required function. It is impossible for a user of the crate to implement IntoTextStyle on their type without this change.

plotters/plotters/src/style/text.rs

Line 40 in 3ad52da

fn into_text_style<P: HasDimension>(self, parent: &P) -> TextStyle<'a>;

Use-case

For an example of where this functionality would be useful, see this example.