Build System: Support `import` declarations up to Ast_core

Context

In JsLIGO, we currently provide an import declaration. But we currently have a nanopass that simply rejects programs containing these declarations.

In the context of the new build system, we would like to support these declarations which requires support in the type checker.

Design

Ast_core will have to be modified to contain these declarations.

Acceptance Criteria

The ability to type check a JsLIGO program with all 3 kinds of import declarations.