Add support for JSON Schema-style additional fields

[myitcv]

In the v0.3.0-alpha6 release we confirmed that the semantics of pattern constraints would remain as is:

The spec recently included a change that proposed to use the JSON schema semantics for pattern constraints ([k]: v), rather than the old CUE semantics. To aid in the transition, cue fmt rewrote these fields to be enclosed in curly braces, which mimics the old semantics under these planned rules.

It has now been decided to keep the old semantics and stop rewriting old uses.

It went on to describe how the JSON Schema concept of additional fields would instead be added:

Instead we now intend to allow the JSON schema semantics additionally by writing [...k]: v. The nice thing here is that any pattern which is to be matched additionally, is now always preceded by a .... It also is a nicer generalization of the syntax.

This issue tracks the changes required for support of additional fields.

[cueckoo]

This issue has been migrated to cue-lang/cue#785.

For more details about CUE's migration to a new home, please see cue-lang/cue#1078.