JsLIGO: enforce `export` keyword

Motivation and Context

Related Issue: #1916 (closed)

Description

This MR works by adding a pass that turns every declaration (which is not marked as private/public) as private. Then, the typer is modified to take into account this attribute when typing.

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

In JsLIGO, the export keyword was not enforced, thus effectively exporting all declarations in namespace.

With this change, the export becomes mandatory to export from namespaces (and top-level .jsligo when imported from another file).

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).