TypeScriptFilter now uses actual file instead of temp (if it exists)

Modified TypeScriptFilter to use the actual file (instead of making a temp one) if the actual file is available (e.g., it's not a StringAsset). This allows the TypeScript compiler to honor TypeScript resource paths (without this change, you can use those which means you have to put all of your TypeScript code in one file).

Also updated the test (TypeScriptFilterTest::testFilterLoad) to cover this new scenario.