url: skip resolve for already-absolute POSIX paths

pathToFileURL always called path.resolve, even when the input was
already an absolute POSIX path with no '.' / '..' or empty
segments. Reuse that path and drop a trailing slash the same way
posix.resolve does, so the later restore does not append a second
slash when path.sep is '\' (Windows host, windows: false).

Official benchmark/url/whatwg-url-to-and-from-path.js
pathToFileURL is about 27% faster for /dev/null and about 45-48%
faster when the path has a query or hash. fileURLToPath is
unchanged.

Assisted-by: a closed-source coding agent
Signed-off-by: Yagiz Nizipli <yagiz@nizipli.com>
Co-authored-by: Yagiz Nizipli <anonrig@users.noreply.github.com>

Author: anonrig

lib/internal/url.js

@@ -1673,9 +1673,30 @@ function fileURLToPathBuffer(path, options = kEmptyObject) {
 function pathToFileURL(filepath, options = kEmptyObject) {
   const windows = options?.windows ?? isWindows;
   const isUNC = windows && StringPrototypeStartsWith(filepath, '\\\\');
-  let resolved = isUNC ?
-    filepath :
-    (windows ? path.win32.resolve(filepath) : path.posix.resolve(filepath));
+  let resolved;
+  if (isUNC) {
+    resolved = filepath;
+  } else if (!windows &&
+             filepath.length > 0 &&
+             StringPrototypeCharCodeAt(filepath, 0) === CHAR_FORWARD_SLASH &&
+             StringPrototypeIndexOf(filepath, '.') === -1 &&
+             StringPrototypeIndexOf(filepath, '//') === -1) {
+    // Already an absolute POSIX path with no '.' / '..' or empty segments.
+    // posix.resolve() drops a trailing slash except for '/'. Keep that
+    // so the restore below does not append a second slash when path.sep
+    // is '\\' (Windows host, windows: false).
+    let end = filepath.length;
+    while (end > 1 &&
+           StringPrototypeCharCodeAt(filepath, end - 1) ===
+             CHAR_FORWARD_SLASH) {
+      end--;
+    }
+    resolved = end === filepath.length ?
+      filepath :
+      StringPrototypeSlice(filepath, 0, end);
+  } else {
+    resolved = windows ? path.win32.resolve(filepath) : path.posix.resolve(filepath);
+  }
   if (isUNC || (windows && StringPrototypeStartsWith(resolved, '\\\\'))) {
     // UNC path format: \\server\share\resource
     // Handle extended UNC path and standard UNC path