Most validation keywords apply to only one of the basic types, so a
"string" type and an "array" type can share a definition without
colliding as a ["string", "array"] type, as long as they don't have any
incompatibilities with each other. Modify UnionTypeFormatter to collapse
these disjoint types into a single definition, without using anyOf.

Change the array.flat() call to use spread notation, since both members are guaranteed to be arrays (unlike the types call, above). Thanks for the suggestion, @domoritz!

Co-authored-by: Dominik Moritz <domoritz@gmail.com>

The vega-lite test originally worked after updating the snapshot, but
there was a broken reference that got mistakenly purged by
removeUnreachable (which assumed that any definition would
only ever have a single property that could hold references).

Added a validation check to vega-lite.test to register the failure,
updated addReachable to check all possible properties (unless
$ref is set; the spec says that everything else gets ignored in
that case).

In addition to the raw schema tests which just check that nothing has
changed since the last snapshot, this adds makeExemplar, which builds
a raw value using the parsed BaseType representation of a TypeScript
type tree. This way, tests can specify valid/invalid data to be verified
using the Ajv validator in addition to raw schema output tests.

In a few simple cases like (1 | number), mergeDefinitions will now
correctly collapse two not-quite-disjoint type definitions together,
by discarding the validation restrictions on the more-restricted type.
This replaces the string-merging logic in UnionTypeFormatter, which
was discarding annotations.