fix: ensure generated routes with leading digits get prefixed correctly

[duniul]

The fix added in https://github.com/TanStack/router/pull/1111 doesn't actually get applied since the replace on line 549 removes any non-alpanumeric characters again. 🙈

This PR moves it to after the final replace and replaces the prefixed _ with a R (for Route) in case the underscore removal is intentional for other reasons. Feel free to change it back if it's safe 👍