Allow themes to have mascots.

[THEGOLDENPRO]

To enrich themes a little more and allow creativity to run wild (+ it would be funny to see anime themed themes), let's add an optional field in theme.toml that allows themes to have a mascot image in their folder.

- kitty
  - theme.toml
  - mascot.png

Something like this... umm.... 🤔

# ... truncated

[metadata]
name = "😺 Kitty"

[pallet]
# ... pallets truncated

[assets]
image.mascot = "./mascot.png"

For an actual full theme example checkout rubber-ducky.

Mascots will be displayed where applications choose to place them for styling. For example in Roseate the rose in the image section screen will be replaced with a mascot if there is one available.

[THEGOLDENPRO]

Should we specify the mascot in a [assets] toml key or should we just look for a supported image file named mascot in the folder (I think the second option is cleaner).

We need to decide this now as future assets may also follow this.

If we choose the [assets] solution that is giving freedom to the user to name the assets anything and personally I dislike that. This throws any kind of naming convention and consistency out the window leading to everyone creating their own version of a naming convention, which leads to confusion for new users. I want the theming system to be pretty beginner friendly.