Cleanup and document module execution order

[aklinker1]

Feature Request

Currently, there's no documentation around the order modules' hooks are executed. Add that and add an E2E test to verify that modules are loaded in the expected order:

1. First, NPM packages
2. Second, "user" modules in the project's modules/ directory
3. Finally, one-off hooks inside your wxt.config.ts

Also need to ensure user modules are executed alphabetically so they can be ordered with filenames.

[aklinker1]

Released in v0.19.12