Disallow adjacent object literals

[szeiger]

Requested by @iuliand-db:

Forbid syntactic sugar that allows omitting the + to merge to objects when both LHS and RHS are dictionary literals. That’s basically never what you want, and usually indicates a comma between the two literals.

Since Jsonnet allows this, simply making it an error is probably not an option, but we could add a warning or a strict mode (neither of which currently exists in Sjsonnet).