Incompatible with Vite >= 3.1.0

Building with Vite > 3.1.0 gives the following error:

```
error during build:
TypeError: Cannot set properties of undefined (setting 'generateBundle')
at configResolved (/.../vite-dts/dist/cjs/plugin.js:56:33)
```

Worked fine prior to 3.1.0 release. Appears to be related to "this" (pun intended): https://github.com/vitejs/vite/commit/2d2f2e5

I believe the fix is to move the `generateBundle` function def outside of the `configResolved` function.