Add deprecation warning when trying to run commands without explicit entrypoint

[Electroid]

In wrangler1, you could run wrangler dev, wrangler publish, etc. without explicitly specifying the entrypoint in the command. Many users will attempt to run these old commands and get confused why they now see the help page. Instead, there should be a deprecation warning that tells them what to do.

[threepointone]

Agreed, we should have explicit errors for when people do common mistakes instead of the generic yargs "expect x arguments, got y" message.

[petebacondarwin]

One can still run wrangler publish with no entry-point in W2, as long as you have defined an entry-point via build.uploads.main. Here is the current output in this case:

> npx wrangler publish
Missing entry-point: The entry-point should be specified via the command line (e.g. `wrangler publish path/to/script`) or the `build.upload.main` config field.

I don't think we need anything else here, right?

[petebacondarwin]

For wrangler dev I think we should also support the build.upload.main field exactly the same as wrangler publish. I will put together a PR to do that, which will also move the bundling into a helper that both commands use.

[Electroid]

For wrangler dev I think we should also support the build.upload.main field exactly the same as wrangler publish. I will put together a PR to do that, which will also move the bundling into a helper that both commands use.

If we support something for publish, it should work for dev too. Similar to #396