Detect entrypoint when one is not provided

[Electroid]

When running wrangler dev, if no entrypoint is provided, wrangler should check for one in the following places.

1. main/module in package.json.
2. build.upload.main in wrangler.toml.

[threepointone]

Alternate proposal - we should only have explicit CLI args for entry. Copied from #258 (comment):

I'm deeply concerned with the ambiguity we've introduced. wrangler dev can now point to either something hidden inside a wrangler.toml file somewhere, or an index.js file locally, or (not implemented yet) the main field in a parent package.json, or (not implemeted yet) the entry field in wrangler.toml, and there's no way to tell without running the command. We should resolve this before we ship to GA.

imo, we should deprecate build.upload.main, not implement the proposed entry field, and not read from package.json. We should only have explicit path in CLI.

[threepointone]

This probably deserves a deeper discussion, let's take it up during the meeting.

[threepointone]

ping

[Electroid]

Deciding against this, for the reasons you outlined.