support detection of non .prettierrc config files

[exectx]

When integrating Tailwind into an existing project that already has a prettier config file that isn't a .prettierrc , an incomplete .prettierrc file is generated.

The current implementation in prettier/index.ts and tailwindcss/index.ts should detect the existing prettier config file and use parseJson or parseScript in case the prettier config is for instance prettier.config.js.

repro: npx sv add tailwind on existing project with a prettier.config.js

Issue #285 is related but focuses mainly on monorepo setups.