update foldering structure for readability and `assets`, `js`

[hanbollar]

Made some foldering changes to help with readability and also to make the repo less overwhelming at first glance. Helps users get started without too much information overwhelm.

foldering changes:

• mrjs-->mrjs-build
• audio --> assets/audio
• biomes --> assets/biomes
• fonts --> assets/fonts
• models --> assets/models
• ui-models --> assets/ui-models
• ecs --> js/systems
• entities --> js/entities
• classes --> js

that is before the file structure was:

root
|- `_unused`
|- `audio`
|- `biomes`
|- `classes`
|- `ecs`
|- `fonts`
|- `models`
|- `mrjs`
|- `ui-models`

after:

root
|- `_unused`
|- `assets`
|    | - audio`
|    | - `biomes`
|    | - `fonts`
|    | - `models`
|    | - `ui-models`
|- `js`
|    | - `entities`
|    | - `systems`
|- `mrjs-build`

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