feat: add File.realPath() to resolve symlinks and normalize paths

Implement a new static method `File.realPath(path)` that resolves the given path by traversing symlinks and removing redundant `./` and `../` components, returning the canonical absolute path. The implementation uses libuv's `uv_fs_realpath` via an asynchronous callback pattern consistent with other File operations. Includes documentation, foreign method declarations, and two error-case tests for nonexistent paths and wrong argument types.

test/io/file/real_path_nonexistent.wren

@@ -0,0 +1,3 @@
+import "io" for File
+
+File.realPath("nonexistent") // expect runtime error: no such file or directory

test/io/file/real_path_wrong_arg_type.wren

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+import "io" for File
+
+File.realPath(123) // expect runtime error: Path must be a string.
+
+// TODO: Write success case tests too when we have an API to create symlinks
+// from Wren.