Add chmod/chmodSync on unix (and fix Cargo.toml) (#1088)

Initial implementation by Srijan Reddy (@srijanreddy98, #672).

js/unit_tests.ts

@@ -12,6 +12,7 @@ import "./read_dir_test.ts";
 import "./write_file_test.ts";
 import "./copy_file_test.ts";
 import "./mkdir_test.ts";
+import "./chmod_test.ts";
 import "./dir_test";
 import "./make_temp_dir_test.ts";
 import "./stat_test.ts";