Build System: Support `import` declarations in Ast_typed

Context

This task is a continuation of #2187 (closed).

In JsLIGO, we currently provide a 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_typed will have to be modified to contain these declarations.

Once #1990 (closed) is closed, the type checker can be modified by simply looking up the signature for the path of the import.

Acceptance Criteria

The ability to type check a JsLIGO program with all 3 kinds of import declarations provided the type checker has the correct persistent environment.