refactor: Use npm packages instead of hardcoded bundles and remove some polyfills

[zusorio]

• Remove the WebRTC adapter.js, it wasn't being used
• Replace the hardcoded bundle of underscore.js with a version from npm
• Remove the dom.js polyfill as it isn't needed in even somewhat modern browsers
• Use the VTTCue and VTTRegion polyfills from the media-captions package and remove the hardcoded ones from vtt.js. This could be further optimized by using some of the parsing from media-captions in the future.
• Remove the fetch polyfill as it's been available in all browsers since 2017
• Re-organize package.json: Since all dependencies are put in the bundle, they should be in devDependencies to prevent users from unnecessarily installing them
• Update webpack config to disable chunkLoading. This is necessary to create a single bundle and was also already done in fix: Allow importing in SSR contexts without error #481
• I haven't replaced the hardcoded vast-client.js bundle yet, as the version from npm seemed to increase the bundle size by 60kb. I think this didn't happen with the vite version, so this is something we can do in the future.