jet: include & extend don't handle absolute paths correctly

Assuming I have a template at foo/baz.jet, {{ include "foo/bar.jet" }} will make resolveNameSibling try to resolve foo/foo/bar.jet before resolving foo/bar.jet: https://github.com/CloudyKit/jet/blob/33cfc27b3e00072655fdb3af24c3253ea5bffb8f/template.go#L153

name will be "foo/bar.jet", sibling will be "foo/baz.jet".

I would like to simplify template resolving & loading and could fix this issue at the same time. Seeing as we are making breaking changes with v3 anyway, I would like to make jet handle paths like most software does:

• ./asd.jet, asd.jet, ../asd.jet will be treated as relative paths and will be resolved relative to the current template’s path
• /foo.jet, /foo/bar.jet will be treated as absolute paths and will be looked up directly in the templates cache and/or the loader, without using the current template’s path

Do you want me to proceed with this? Did I miss something?