Update React tsconfigs to use new JSX transform

[teddybradford]

For the eslint config boilerplate:

• Use flat configs
• Add jsx-runtime flat config
• Use .ts extension for config file (the Jiti requirement is already a Vite dep)

For React tsconfigs:

• Switch JSX transform from "preserve" to newer "react-jsx" (allows developers to omit React imports in component files)
• Remove unnecessary React imports from boilerplates

[magne4000]

@teddybradford thank you for this contribution!