Updated type definitions for handlebars#registerPartials (#27388)

* Updating types for #registerPartial

#registerPartial is a polymorphic function now it accepts a single object making the second param optional.

Excerpt from the implementation:

```js
  registerPartial: function(name, partial) {
    if (toString.call(name) === objectType) {
      extend(this.partials, name);
    } else {
      if (typeof partial === 'undefined') {
        throw new Exception(`Attempting to register a partial called "${name}" as undefined`);
      }
      this.partials[name] = partial;
    }
  },
```

* Updated Handelbars#registerPartials

Removed extra spaces

* Added function overload with typed arguments

Replaced polymorphic argument types for registerPartial function with overloaded function definition

types/handlebars/index.d.ts

@@ -40,6 +40,7 @@ declare namespace Handlebars {
     export function unregisterHelper(name: string): void;
 
     export function registerPartial(name: string, fn: Template): void;
+    export function registerPartial(spec: { [name: string]: HandlebarsTemplateDelegate }): void;
     export function unregisterPartial(name: string): void;
 
     // TODO: replace Function with actual signature