This was easier to fix than expected. The fix should allow users to have their own `_redirects` file in the project root. If this file exists, it gets copied to `build/` before `adapter-netlify` appends the catchall rule to it.

This behaviour is [already documented](https://github.com/sveltejs/kit/tree/master/packages/adapter-netlify#using-netlify-redirect-rules), but [currently broken](sveltejs#1585).

How can I write an automated test for this adapter?

I did the following to test this manually:

1. Use `yarn link "@sveltejs/kit"` and `yarn link "@sveltejs/adapter-netlify"` in my application repository to point the two node modules to my modified versions.
2. Run `svelte-kit build` with a `_redirects` file in the project root.
3. Check if content of `build/_redirects` is expected.
4. Run `svelte-kit build` *without* a `_redirects` file in the project root.
5. Check if content of `build/_redirects` is expected.

Any guidance or help how to match the desired code quality to get this PR merged is appreciated.

close sveltejs#1585