Transform namespaces declared with module keyword to namespace keyword in d.ts files

[CC972]

Addresses part of #51825.

With this PR, we encourage people to move away from declaring namespaces with the module keyword.

Currently, if we have foo.ts:

module foo {}

This leads to the following foo.d.ts file:

declare module foo {}

This PR changes the declaration emit to transform module to namespace:

declare namespace foo {}

"Ambient module declarations", such as the below, will be unaffected:

declare module "bar" {}

[typescript-bot]

The TypeScript team hasn't accepted the linked issue #51825. If you can get it accepted, this PR will have a better chance of being reviewed.

[sandersn]

1. This issue doesn't fix all of Consider deprecating namespaces declared with the module keyword #51825, so I'd recommend swapping the wording to "Addresses part of Consider deprecating namespaces declared with the module keyword #51825" so that github doesn't close the original issue.
2. The implementation seems straightforward and the test changes look right to me, but @rbuckton should probably sign off on the API change.

[typescript-bot]

This PR doesn't have any linked issues. Please open an issue that references this PR. From there we can discuss and prioritise.