Fix generated namespace types in index.d.ts

[robpalme]

Previously some namespace types were generated using the legacy module keyword from ten years ago. Now we emit the modern TS namespace keyword that has been the preferred keyword since TypeScript 1.5 in 2015.

Please review the resulting diff for the generated dist/index.d.ts file: https://gist.github.com/robpalme/a320dc3f0cb50bcd14962bca46827dae/revisions

Note that the outer Ambient Module Declaration intentionally remains untouched because it is not a namespace. These are differentiated by using a quoted string rather than a bare identifier

module "quoted" {
vs
module bare {

---

Background: This usage of the legacy keyword was found by a TypeScript real-world test suite that checks the compatibility of proposed changes. Using the module keyword for namespaces is proposed for deprecation in TypeScript 6.0 so it's worth getting ahead of this.

[toji]

Thank you!