Make WASM use IndexedDB for data persistence

[Semphriss]

This PR makes WASM builds use IndexedDB in browsers, which allows much larger files to be stored. It also uses data persistence, which prevents browsers from automatically deleting progress without warning.

Custom levels done in the browser will now be saved, and the warning prompting users to download their levels will only show if data persistence has been denied.

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NOTE: Data persistence requires special permission from the browser. While searching, I found that although Firefox does this normally and asks users if they want data to persist, Chrome and Chromium-based browsers (Edge, Opera, Brave, etc.) decide by themselves which websites they consider important, and will never prompt the user to ask for preference.

Since users using those browsers will never be able to choose if they want their data saved or not, I added a prompt to users of those browsers directing them to use Firefox as a solution.

Info can be found here: https://web.dev/persistent-storage/#how-is-permission-granted