build: fix SvelteKit CSP 'stric-dynamic' issue in Firefox

[peterpeterparker]

Motivation

Using a CSP 'strict-dynamic' with SvelteKit breaks in Firefox.

Solution

Extracting the script content that gets injected by SvelteKit in index.html into a separate JS script that gets appended with a script loader.

Exact same "script loader" as the one we use to use for Rollup.

Issue

• SvelteKit strict-dynamic CSP support sveltejs/kit#3558

Changes

• update build.csp.mjs to extract script
• add script loader to app.html

Screenshot

[lmuntaner]

I few comments to better explain what's going on and to avoid us to remove some of these in the future.

But already approving.