Add watch script to watch all packages at once

[planger]

I often find myself adding that in Theia-based projects, especially for those that have multiple extensions.

[vince-fugnitto]

Thank you for the contribution!
Do you believe watch should be documented, especially for new developers who are likely to use the generator, or build their own extensions for the first time? @JonasHelming any thoughts?

[JonasHelming]

Yes, I guess we should add it here as a second option alternativly to watching both separatly https://github.com/eclipse-theia/generator-theia-extension/blob/master/templates/README.md#developing-with-the-browser-example

[planger]

Thanks for the feedback! Good idea with adding the global yarn watch to the README, I made a suggestion in a new commit 0657143 in this PR.

[vince-fugnitto]

The changes look good thank you! Can you please squash so we can merge?

I'm a little confused as to why the docs describe the watch workflow in such a way where we need to launch the backends with vscode, we can alternatively also describe how to do so simply with the terminal.

[planger]

Thanks a lot! I force-pushed a single commit with all changes.

I'm a little confused as to why the docs describe the watch workflow in such a way where we need to launch the backends with vscode, we can alternatively also describe how to do so simply with the terminal.

I agree that it would make sense to at least also mention that the backend can be started from within VSCode via the launch config or with e.g. cd browser-app; yarn start. Do you want me to open a separate PR with a proposal?

[vince-fugnitto]

Thanks a lot! I force-pushed a single commit with all changes.

I'm a little confused as to why the docs describe the watch workflow in such a way where we need to launch the backends with vscode, we can alternatively also describe how to do so simply with the terminal.

I agree that it would make sense to at least also mention that the backend can be started from within VSCode via the launch config or with e.g. cd browser-app; yarn start. Do you want me to open a separate PR with a proposal?

If you'd like to and agree it adds value for extension developers then please feel free to open the PR :)

[JonasHelming]

I created a ticket for the follow up so we can merge this for now