add more special cases to ts namespace transform

[evanw]

With this PR, esbuild's TypeScript-to-JavaScript transform will no longer omit the namespace in this case:

namespace Something {
  export declare function Print(a: string): void
}
Something.Print = function(a) {}

This was previously omitted because TypeScript omits empty namespaces, and the namespace was considered empty because the export declare function statement isn't "real":

namespace Something {
  export declare function Print(a: string): void
  setTimeout(() => Print('test'))
}
Something.Print = function(a) {}

The TypeScript compiler compiles the above code into the following:

var Something;
(function (Something) {
  setTimeout(() => Print('test'));
})(Something || (Something = {}));
Something.Print = function (a) { };

Notice how Something.Print is never called, and what appears to be a reference to the Print symbol on the namespace Something is actually a reference to the global variable Print. I can only assume this is a bug in TypeScript, but it's important to replicate this behavior inside esbuild for TypeScript compatibility.

The TypeScript-to-JavaScript transform in esbuild has been updated to match the TypeScript compiler's output in both of these cases.

Fixes #1158