This PR adds a customized version of the Sentry Vite plugin to the SvelteKit SDK.

After realizing that the proposed API in #7787 doesn't work as I originally thought, this PR also re-reworks it a little bit:

• Instead of having a super plugin that adds our other plugins (this doesn't work), we now export a factory function that returns an array of plugins. SvelteKit also creates its plugins this way.
• The currently only plugin in this array is the customized Vite plugin.

The customized Vite plugin differs from the Vite plugin as follows:

• It only runs on builds (not on the dev server)
• It tries to run as late as possible by setting enforce: 'post'
• It uses the closeBundle hook instead of the writeBundle hook to upload source maps.
  • This is because the SvelteKit adapters also run only at closeBundle but luckily before our plugin
• It uses the configure hook to enable source map generation
• It flattens source maps before uploading them
  • We to flatten them (actually sorcery does) to work around weird source maps generation behaviour.

Also:

ref #7669