language: support for equivalent of JSON Schema's additionalProperites

[myitcv]

Is your feature request related to a problem? Please describe.

JSON Schema supports the notion of additionalProperties (JSON Schema spec):

The additionalProperties keyword is used to control the handling of extra stuff, that is, properties whose names are not listed in the properties keyword or match any of the regular expressions in the patternProperties keyword. By default any additional properties are allowed.

The value of the additionalProperties keyword is a schema that will be used to validate any properties in the instance that are not matched by properties or patternProperties. Setting the additionalProperties schema to false means no additional properties will be allowed.

CUE currently supports the equivalent of patternProperties via pattern constraints.

But as has been previously noted, pattern constraints do not allow us to encode the semantics of additionalProperties.

Describe the solution you'd like

Support for JSON Schema's additionalProperties. As has been previously suggested, this could look like:

[...Pattern]: expression

Describe alternatives you've considered

No means of doing this.

Additional context

n/a