Check process.cwd before calling it

Some runtimes (like React Native) has a `process` global, but doesn't provide a `cwd` function. Instead of assuming the `cwd` function is available, just because the `process` global is, merging this PR will update the check to actually verify the `process.cwd` is a callable function before relying on it.

src/path.ts

@@ -81,7 +81,7 @@ export const join: typeof path.join = function (...arguments_) {
 };
 
 function cwd() {
-  if (typeof process !== "undefined") {
+  if (typeof process !== "undefined" && typeof process.cwd === "function") {
     return process.cwd().replace(/\\/g, "/");
   }
   return "/";