Factor out common types that don't depend on the exact schema into schema-dts-lib

[Eyas]

This will be a breaking change for schema-dts-gen since the output of the generator will no longer be self contained, and will start referencing schema-dts-lib which needs to be imported by the user.

schema-dts itself is updated to start importing the library, so this is self contained for it.

We can't move WithContext, Graph, or SchemaValue out of our helper types though:

• WithContext and Graph define the context specifically as "https://schema.org" which is not true for all schemas
• SchemaValue depends on Role<> when Role is available/emitted

[Eyas]

@cochinescu this is basically what I have in mind.

The new merging utils would go here as well. WDYT?

[cochinescu]

Yes, keeping WithContext, Graph, and SchemaValue in helper_types.ts while moving the schema-agnostic pieces into schema-dts-lib seems like the right split. The new merge utilities should fit there as well.

Once #213 lands, I can update #210 to move MergeLeafTypes and its internal helpers into schema-dts-lib while keeping the leaf export change in the generator. If you’d prefer, I can also stack that work on top of newlib before this merges, please lmk.