Separate `hotRestart` and `hotReload` into two methods each in the embedder

[srujzs]

Currently, hotRestart and hotReload are single entrypoints in the embedder for the DDC library bundle format.

This presents issues when it comes to debugging and deferring execution, as we don't have a mechanism to pause execution, load new sources, delete and reregister breakpoints, and then continue execution in DDC.

With hotRestart, this means main runs before we get a chance to reregister breakpoints. With hotReload, this means execution continues before we can reregister breakpoints.

Instead they should be split into two methods in the embedder each:

• hotRestartBegin: Downloading and loading of sources and any reinitialization in the embedder.
• hotRestartEnd: Calls main after a previous hotRestartBegin call.
• hotReloadBegin: Downloading and loading of sources.
• hotReloadEnd: Reinitialization, which should continue execution but with the new libraries.

[srujzs]

FYI @Markzipan @nshahan

I believe with Mark's change to defer main execution we don't need the first half of this request.