Option to have index.tsx as the default behavior of routes instead of route.tsx

[ferretwithaberet]

I've seen quite some people struggle with the default nesting behavior of routes. It is also quite confusing that routes nest by default in my opinion.
I propose an option for the router plugin to consider routes as index.tsx by default. It could be named defaultRouteBehavior and possible options would be route or index. The default value of this option would be route, for backwards compatibility, but it could be changed to index in a major version to make nesting explicit instead of implcit to avoid confusion.

Setting this option as index will cause routes like:

- some-route.tsx
- some-route.other-route.tsx

to be treated like

- some-route.index.tsx
- some-route.other-route.index.tsx

instead of the default behavior of

- some-route.route.tsx
- some-route.other-route.route.tsx