Add flag to avoid trailing slash in prefix

[jennyEckstein]

#122
Add flag prefixAvoidTrailingSlash to avoid appending trailing slash to specified prefix.

Checklist

• run npm run test and npm run benchmark (there is no benchmark script)
• tests and/or benchmarks are included
• documentation is changed or added
• commit message and code follows Code of conduct

[mcollina]

Great work! Would you mind to add a test for the new type as well as a few of new test for the option (instead of changing existing ones!). Thanks!

[jennyEckstein]

Great work! Would you mind to add a test for the new type as well as a few of new test for the option (instead of changing existing ones!). Thanks!

The test, that I altered was failing with my changes, since previously /static would have returned "Not Found" and now it returns 200. plugginOptions for "register /static" block now specifies prefixAvoidTrailingSlash = true, I thought of repurposing the test.
I can create a separate category for the option, to make it more clear. Thank you for a good suggestion.

[mcollina]

Great work! Could you add a test for the type in test/types/index.ts? Thanks

[mcollina]

lgtm

[delvedor]

LGTM