Draft: Initialise Flow Builder Frontend Island

What does this MR do and why?

Initialise Flow Builder Frontend Island

Prettify json file

Copy vite_config from Duo Next to create custom element JavaScript

Rename flow editor to flow builder

Update CSS in Island

Add build:watch command

Replace custom styles with TW

Remove comma in prettierrc

Setup vitest for unit testing

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

MR acceptance checklist

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Related to #578329