Enhancement: Strike through and underline

[RobertBrewitz]

With parley coming into bevy_vello we can add underline and strike-through, parley only returns the instructions where to draw the lines, so we need to implement the rendering.

We would add the follow fields on VelloTextStyle:

    pub underline: bool,
    pub underline_offset: Option<f32>,
    pub underline_size: Option<f32>,
    pub underline_brush: Option<Brush>,
    pub strikethrough: bool,
    pub strikethrough_offset: Option<f32>,
    pub strikethrough_size: Option<f32>,
    pub strikethrough_brush: Option<Brush>,

Depends on #130

[simbleau]

I'd say to 99% of our users, anything other than underline: bool and strikethrough: bool is unlikely to be used. However I suppose the other fields are a nice benefit. +1

[simbleau]

Something else: What about a blinking cursor, how would we support that, similarly?