Add custom scrollbar to theme

[ChiguireDigital]

Summary

• Add a custom hand-drawn scrollbar to the UI theme.
• Replace the default scrollbar appearance with custom pixel art assets.
• The Storybook currently benefits from this custom scrollbar.

Resolves #2346

[ChiguireDigital]

this is how it looks for now after your feedback, what do you think?

[github-actions]

Play this branch at https://play.threadbare.game/branches/ChiguireDigital/add-custom-scrollbar/.

(This launches the game from the start, not directly at the change(s) in this pull request.)

[manuq]

I love it!

Maybe the scroll can look a bit more "hand drawn" if the line is more erratic, like the banner border next to it:

For that, you can play with the texture margins and axis stretch settings, so the middle part can be tiled. See how the buttons do it:

Let me know if you agree and want to do as suggested, otherwise I'll merge it as-is.

[ChiguireDigital]

Done!! This is the result with the fine line, also changed the grabber a bit.