I started with allowImportingTsExtensions and then realised some where not enough strict and not homogenous at all.
Changes:

• Target
  • Set to ES2020 to match Vite default target, enable useDefineForClassFields because only implied if target > 2022. This avoid TS autocompleting new features that would not be widely supported like intl.formatRange or array.at. Probably we can bump some numbers for Vite 5 to include string.replaceAll and add ES2021.string to the list
  • Always include dom.iterable
• Bundler mode
  • allowImportingTsExtensions and update template accordingly
  • remove allowSyntheticDefaultImports and allowSyntheticDefaultImports: Personally I don't like using a default export when the lib doesn't explicitly set one and expecting it to work because bundler are doing crazy amount of interop.
• Linting
  • Using strict + noUnusedLocals + noUnusedParameters, I hope no debate here
  • noFallthroughCasesInSwitch is a better default because it's too easy to miss.
  • noPropertyAccessFromIndexSignature: I started using it recently and I quite like it. I'm hesitant to add it
  • noImplicitReturns: I stopped enabling this because TS correctly infer undefined on empty branches and it's annoying when having early return on callback code (like we do in Vite plugins for ex).
  • useUnknownInCatchVariables: could be added but it's really annoying to not be able to set the error type to AxiosError for ex.

Note: I did not touch the svelte config because it already extend the one from the svelte team (and I know they don't use TS the same way as I do at all)