Add import destructuring and amends/extends of classes within modules

[HT154]

Destructuring import clauses

Only supports types and properties. Methods are not supported.

Usage

// module members may be aliased as well
import "foo.pkl" as { bar, baz as qux }
// module itself may be imported with an alias, which must come first
import "foo.pkl" as myFoo, { bar, baz as qux }

amends/extends module members

Usage

extends Foo in "bar.pkl"
amends Foo in "bar.pkl"

Class and property members may be amended

Note

Non-module classes do not have an output: ModuleOutput property and thus modules extending/amending them cannot be evaluated directly using the CLI. Such modules may still be imported and used by other modules.

Closes #966