Signatures: preserve them in `ast-typed`

Motivation and Context

There was no signature declaration (D_signature) in ast_typed. As a result, because build works in terms of ast_typed, when we imported a file, we lost the signatures declared in it.

Description

In this MR we add D_signature to the ast-typed and make sure it is preserved. Also export is enforced for exporting signatures in JsLIGO.

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

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).