V5: Theme/Generator Overhauls

[endigo9740]

Describe the feature in detail (code, mocks, or screenshots encouraged)

We're currently aware of several areas of improvement for both themes and the Generator app. We'll track these below.

Theme Format

• Bugfix: https://github.com/skeletonlabs/skeleton/issues/3417
• Bugfix: https://github.com/skeletonlabs/skeleton/issues/3517
• [ ] Feature: text highlight color

Generator Features Request

• [ ] Refactor and cleanup the app structure and state management
• [ ] Improved documentation and tutorials within the app itself
• [ ] Consider the use of Superforms and Zod for form handling.
• [ ] Feature: custom font previews via FontSource APIs (or similar)
• [ ] Feature: background image and mesh gradient previews
• [ ] Implement v4 theme format migration

What type of pull request would this be?

Enhancement

Provide relevant links or additional information.

• Old Generator Thread: https://github.com/skeletonlabs/skeleton/issues/1142
• Make considerations for: https://github.com/skeletonlabs/skeleton/issues/3465
• Inclusive color contrast: https://typefully.com/DanHollick/wcag-3-and-apca-sle13GMW2Brp

[endigo9740]

Per the recent announcement, these changes are being bump to v5. We'll return to them as soon as the new component updates in v4 are out the door.