[now-routing-utils] Change behavior of trailingSlash: true redirects

[styfle]

This PR changes the behavior of trailingSlash: true after we received feedback that files should not be redirected with a trailing slash. This matches the behavior of serve and serve-handler.

Examples

• /index.html => serve
• /assets/style.css => serve
• /assets => redirect to /assets/
• /assets/style => redirect to /assets/style/

Additional

In order to avoid duplicate content, this PR also adds redirects to files without a trailing slash.

• /about.html/ => redirect to /about.html
• /assets/style.css/ => redirect to /assets/style.css

Fixes #3731

[dav-is]

Amazing work 🙌

[TooTallNate]

Seems like the now dev tests are failing:

  ✖ [now dev] test trailingSlash true serve correct content Rejected promise returned by test

  1 test failed

  [now dev] test trailingSlash true serve correct content

  /Users/runner/runners/2.164.0/work/now/now/packages/now-cli/test/dev/integration.js:144

   143:     const actualText = await res.text();              
   144:     t.is(actualText.trim(), expectedText.trim(), msg);
   145:   }                                                   

  Rejected promise returned by test. Reason:

  TypeError {
    message: 'expectedText.trim is not a function',
  }

[styfle]

@TooTallNate I forgot a parameter in the tests 🤦‍♂

(if our tests used typescript it would have caught that mistake 😄 )