Add "Handlebars.VM.resolvePartial" to type definitions

- Handlebars.VM is actually not part of the API,
  but Handlebars.VM.resolvePartial is mentioned
  in the documentation and is thus now treated
  as part of the API.

Closes #1534

lib/handlebars/runtime.js

@@ -209,6 +209,9 @@ export function wrapProgram(container, i, fn, data, declaredBlockParams, blockPa
   return prog;
 }
 
+/**
+ * This is currently part of the official API, therefore implementation details should not be changed.
+ */
 export function resolvePartial(partial, context, options) {
   if (!partial) {
     if (options.name === '@partial-block') {