feat(turbo): add main pointing to bin (#5333)
### Description
<!--
✍️ Write a short summary of your work.
If necessary, include relevant screenshots.
-->
Add a `package.json` `main` filed so that `require.resolve` works.
### Testing Instructions
```
// before
require.resolve('turbo');
// Throws module not found error
// After
require.resolve('turbo');
// /path/to/turbo/bin/turbo
```
Many tools rely on `require.resolve` to find where a package was
installed. Adding this field to the cli package shouldn't change any
behaviors when using the package in currently documented ways. The only
downside here is that this package does not export javascript, so this
will mean that someone require/import'ing the package will get a
different error. AFAIK there is no other reliable way to find where on
disk a package was installed than `require.resolve`, so `main` being
missing would have errored for those folks anyway.
Co-authored-by: Alexander Lyon <arlyon@me.com>
