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

[CC972]

Fixes microsoft#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" {}