[legacy-framework] fix route manifest for mdx pages

[flybayer]

What are the changes and their implications?

Fix a bug where mdx files caused the route manifest generation to fail the build.

Now the route manifest for mdx files is a PascalCase version of the route path instead of trying to parse a default export name.

mdx routes:
/               -> Routes.Index()
/home           -> Routes.Home()
/docs/two-words -> Routes.DocsTwoWords()

Bug Checklist

• Integration test added (see test docs if needed)