`bevy_gizmos`: Expand text gizmos character support beyond ASCII

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

The text gizmos stroke font only supports ASCII characters.

## What solution would you like

Add more unicode characters: extended latin characters, CJK characters, simple emojis, etc.

The way `StrokeTextLayout` maps UTF-8 codes to glyphs would need to be changed. At the moment we only support ASCII, so we just use the ASCII code itself to index into the glyphs array. This wouldn't be possible with expanded character support.

Start with the `bevy_gizmos` text-gizmo implementation, especially `StrokeTextLayout` and the glyphs array. Trace how UTF-8 characters are currently mapped to ASCII indices, then determine an expanded mapping suitable for extended Latin, CJK, and simple emoji characters. Done means supported non-ASCII characters render correctly instead of being limited to ASCII.

Written by the indexing model from the issue text.