File.realPath().

Not tested yet, because we can't create symlinks from Wren and I don't
want to check symlinks into the repo, but it seems to do the right
thing.

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

@@ -0,0 +1,6 @@
+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.