feat: drop support for legacy module namespaces

[acutmore]

Context

microsoft/TypeScript#51825
nodejs/amaro#168

Description

This PR is a breaking change.
Continuing on from #32 but instead of allowing more ModuleDeclaration AST nodes, this PR removes support for erasing namespace declarations that use the deprecated module keyword.

Still erased (no error emitted)

declare namespace N1 {}
namespace N2 {}
declare global {}
declare module "./path" {}

No longer erased (triggers onError callback)

declare module M1 {}
module M2 {}

[robpalme]

Just a doc issue