fix(generators): use `export type` vs `export`

[b-steel]

Summary

Change export { NextFunction } to export type { NextFunction } and change authentication generator to look for either version when inserting new code.

• [x] Tell us about the problem your pull request is solving. I get the following error in src/declarations.ts on the line with export { NextFunction }: Re-exporting a type when 'isolatedModules' is enabled requires using 'export type'.ts(1205).

  If that suggestion is followed, then running feathers generate authentication fails since it is looking for the line export { NextFunction } to insert new code after/before.

• [x] Are there any open issues that are related to this? Not that I can find

• [x] Is this PR dependent on PRs in other repos? No

[daffl]

I believe this should work and I'm not entirely sure why the tests are failing. Could you merge with latest and see if that works?

[b-steel]

@daffl Yeah I was a bit confused as well, the error from the tests was rather cryptic. I merged the latest and it seems to be passing now.

[daffl]

Thank you! Will go out with the next release.