fix(remote): define special variables behavior

Issue #2267 — Define semantics of file-path special variables when the
Taskfile is loaded from a remote source (HTTP/HTTPS/Git):

- TASKFILE / ROOT_TASKFILE: raw URL (fixes the broken `https:/...`
  caused by filepath.Join collapsing the double slash)
- TASKFILE_DIR / ROOT_DIR: empty string — a DIR variable cannot point
  to a URL
- TASK_DIR: resolved against USER_WORKING_DIR

Export taskfile.IsRemoteEntrypoint so the compiler can dispatch on the
nature of the entrypoint without relying on `c.Dir == ""` (a side
effect of the remote path).

taskfile/node.go

@@ -73,7 +73,7 @@ func NewNode(
 	return node, err
 }
 
-func isRemoteEntrypoint(entrypoint string) bool {
+func IsRemoteEntrypoint(entrypoint string) bool {
 	scheme, _ := getScheme(entrypoint)
 	switch scheme {
 	case "git", "http", "https":