src: refactor embedded entrypoint loading

[joyeecheung]

This patch:

1. Refactor the routines used to compile and run an embedder entrypoint. In JS land special handling for SEA is done directly in main/embedding.js instead of clobbering the CJS loader. Add warnings to remind users that currently the require() in SEA bundled scripts only supports loading builtins.
2. Don't use the bundled SEA code cache when compiling CJS loaded from disk, since in that case we are certainly not compiling the code bundled into the SEA. Use a is_sea_main flag in CompileFunctionForCJSLoader() (which replaces an unused argument) to pass this into the C++ land - the code cache is still read directly from C++ to avoid the overhead of ArrayBuffer creation.
3. Move SEA loading code into MaybeLoadSingleExecutableApplication() which calls LoadEnvironment() with its own StartExecutionCallback(). This avoids more hidden switches in StartExecution() and make them explicit. Also add some TODOs about how to support ESM in embedded applications.
4. Add more comments