Add trailingSlash option to redirect requests

[fvsch]

See https://bjornlu.com/blog/trailing-slash-for-frameworks

• If people use servitsy to serve content that would be served by GitHub pages, Cloudflare Pages, Netlify or some other host, having some control over whether URLs contain trailing slashes or not might be necessary to be close enough to prod.
• The behavior used by Cloudflare Pages is pretty good, I might try to make it the default.

I’m thinking something like:

trailingSlash value	When matching filename.html	When matching folder or folder/index.html
off	Do nothing	Do nothing
auto (default)	Redirect /filename/ to /filename	Redirect /folder to /folder/
always	Redirect /filename to /filename/	Redirect /folder to /folder/
never	Redirect /filename/ to /filename	Redirect /folder/ to /folder