* caddyfile: Add parse error on site address in `{`
This is an incredibly common mistake made by users, so we should catch it earlier in the parser and give a more friendly message. Often it ends up adapting but with mistakes, or erroring out later due to other site addresses being read as directives.
There's not really ever a situation where a lone '{' is valid at the end of a site address (but I suppose there are edgecases where the user wants to use a path matcher where it ends specifically in `{`, but... why?), so this should be fine.
* Update caddyconfig/caddyfile/parse.go