Move build and update commands to the core

[fson]

Move build:* and update:* command placeholders to the eas-cli core package. Remove webhook:* placeholders.

Why?

Plugins are a useful feature of oclif, they can be used to create extensions to the CLI that can be installed separately even if they're not a part of the default installation and to even share commands between many CLIs. They're also a way to split a CLI into modular components.

However, build and update commands should be in the core package and not be separate plugins (at least for the time being), because:

• These are core functionality that's included in the CLI by default, they're not "add-ons".
• Having them as separate packages adds unnecessary maintenance overhead (more configuration, more packages to publish) and slows down tools like tsc.
• We can create libraries like @eas/config for functionality that's shared between packages.

I'm also removing the webhook:* command stubs for now. I added them as examples initially, but currently I'm not certain, if we need to have these commands for editing webhooks, as a website UI seems like a better experience for this task.

[byCedric]

If you think this is a good way forward, let's do it! I'm not sure if we fully benefit from the composability of OCLIF, but we can change it later if we want or need it. 😄