Provide a warning when an externalConsole attribute used in Launch

[sandy081]

Testing #11091

Steps to Reproduce:

Since externalConsole attribute is deprecated, it would make sense to show a warning in the launches when this attribute is used.

[isidorn]

August to investigate

[isidorn]

Fixed by 74a526a in https://github.com/Microsoft/vscode-node-debug

[isidorn]

Actually that is not a good fix, since we still want to show the message that the attribute is deprecated.
@aeschli we want to always show a custom warning message for a field in our json schema. I figured we can use a pattern that always fails and use the errorMessage there. But not sure if there is some nicer way to achieve this?

[aeschli]

I would do the 'pattern' and 'custom error' message trick. JSON schema doesn't have a way to disallow a specific property. You can enforce that only the specified properties are allowed, but the error message will be generic.

[isidorn]

Work on our side is done, for this to fully work we are waiting for #11305

[sandy081]

@isidorn It seems the warning is appearing on the value instead of the attribute. It looks like as if the value is not supported. But in this case attribute itself is not valid. So does it makes sense to show squiggle on attribute or complete attribute-value?

[isidorn]

@sandy081 It would be nicer to show the warning on the attribute, but this is not currently supported by @aeschli. So we are doing this as a workaround