Do not use declare module in first-party type declarations

First-party type declarations should simply be provided at top level
in .d.ts files with the right filenames.

The `declare module` was breaking usage like `import
"handlebars/runtime.js"` (which happens to be the only way to import
the runtime from a project with native ES modules, "type": "module").

Signed-off-by: Anders Kaseorg <andersk@mit.edu>

runtime.d.ts

@@ -1,5 +1,3 @@
-import Handlebars = require('handlebars')
+import Handlebars = require('handlebars');
 
-declare module "handlebars/runtime" {
-
-}
+export = Handlebars;

types/index.d.ts

@@ -270,8 +270,4 @@ export interface Logger {
 
 export type CompilerInfo = [number/* revision */, string /* versions */];
 
-declare module "handlebars/runtime" {
-  export = Handlebars;
-}
-
 export default Handlebars;