fix regex for pages that end with index

[fridays]

Client navigation currently fails if the filename of a page ends with *index
It happens in both v2 and v3

Reproduce the issue:

// pages/index.js
import Link from 'next/link'
export default () => <Link href='/abcindex'><a>go</a></Link>

// pages/abcindex.js
export default () => <div>abc</div>

1. Open /
2. Click the link
3. See 404 error instead of page

This PR fixes it by changing two regular expressions.

[fridays]

Before this received /abcindex and produced /abc
Now it only matches /index or /abc/index

[fridays]

Before this receiced abcindex and produced /abc
Now it only matches index or abc/index
And it does that after the windows slash conversion

[arunoda]

This fix looks solid. Thanks.