Add support for app-level code

[ejizba]

Users can specify an entry point file as the main field in their package.json and we will load the file during worker initialization. The file can be used to run any arbitrary app-level code (as opposed to function-level code), but I expect the most common scenario is to register hooks and eventually functions.

A few notes:

• I handle commonjs vs es6 the exact same way we already do it for function-level files
• The npm docs say "If main is not set it defaults to index.js in the package's root folder.", but I worry users may have a random "index.js" field in the root of their app already because it's such a common name. Instead, I decided to make the feature opt-in only, meaning users have to explicitly add the main field if they want the entry point file to be loaded

Fixes #537

[hossam-nasr]

Looks good! Just left a few comments/questions