Add four new file system checking functions: Stat.isFile and Stat.isDirectory for querying entry type from a Stat object, and File.exists and Directory.exists static methods that return booleans by attempting to stat the path and checking the appropriate type. Also bump MAX_METHODS_PER_CLASS from 12 to 14 to accommodate the new Stat foreign methods, and clean up unused Scheduler imports in existing stat tests.
Implement asynchronous stat() on File class returning a new Stat object with device, inode, mode, linkCount, user, group, specialDevice, size, blockSize, and blockCount fields populated from libuv uv_fs_stat callback. Register foreign method statPath_ in module registry, bump MAX_METHODS_PER_CLASS to 10, add Stat class to io.wren with indexed field accessors, update documentation index with Stat page, and include test coverage for stat on existing file and nonexistent path.
