File import: make `export` mandatory to export values/types/namespaces when importing modules from other files

Motivation and Context

Related Issue: #2003 (closed) .

Description

We make type-checking with signature filter only exported values.

As this is a breaking change, our libraries should be checked after this change.

Component

• compiler
• website
• webide
• vscode-plugin
• debugger

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Performance improvement (non-breaking change that improves performance)
• None (change with no changelog)

Changelog

When importing a file with #import, every definition was imported, even those not marked with export explicitly in JsLIGO, e.g. in previous versions we had:

$ ligo compile expression jsligo y --init-file modules_export_importer.jsligo
42

$ cat modules_export_imported.jsligo 
type t = int;

const x : t = 42;
$ cat modules_export_importer.jsligo 
#import "modules_export_imported.jsligo" "M"

const y : M.t = M.x;

After this change, export is needed in JsLIGO to effectively export a declaration:

$ ligo compile expression jsligo y --init-file modules_export_importer.jsligo     
File "modules_export_importer.jsligo", line 3, characters 10-13:
  2 | 
  3 | const y : M.t = M.x;

Type "t" not found. 

The same applies for CameLIGO: after this change, declarations marked with @private are not exported.

Checklist:

• If a new syntax has been introduced, put a message on slack ligo-lsp
• Changes follow the existing coding style (use dune @fmt to check).
• Tests for the changes have been added (for bug fixes / feature).
• Documentation has been updated.
• Changelog description has been added (if appropriate).
• Start titles under ## Changelog section with #### (if appropriate).
• There is no image or uploaded file in changelog
• Examples in changed behaviour have been added to the changelog (for breaking change / feature).