`bevy_gizmos`: Stroke font loading support

## What problem does this solve or what need does it fill?

At the moment the stroke font we use is just baked into the text gizmos implementation.

Users are able to create their own stroke font data, store it in a `StrokeFont`, and generate a text layout using the `render` function`, but it's a bit awkward and obscure.

Also the `text` and `text_2d` functions only support the built-in font, so users would have to create their own alternatives with a param for the custom font or something.

## What solution would you like?

Support for loading stroke fonts. Add an option to specify a font to `GizmoConfig`.

We'd also need to decide on a format for stroke fonts, there isn't a standard. At the moment we just store a list of points, where strokes are ranges into the list of points, and glyphs are ranges into the list of strokes.