(GH-1413) Resolve promise correctly in Folding feature (#1416)

Previously in PR #1414 the Folding provider was updated, however the promise
resolution was incorrect.  This caused the grammar load to never trigger the
Then command.  This commit instead uses the correct way to return the value
for the promise and, when then triggers the Then statement correctly.

Author: glennsarti

src/features/Folding.ts

@@ -545,8 +545,8 @@ export class FoldingFeature implements IFeature {
         // Branching for the different vscode-textmate modules
         if ("loadGrammarFromPathSync" in registry) {
             // V3 of the module allows synchronous loading of a grammar
-            return new Promise( (grammar) => {
-                return registry.loadGrammarFromPathSync(grammarPath);
+            return new Promise( (resolve) => {
+                resolve(registry.loadGrammarFromPathSync(grammarPath));
             });
         } else {
             // However in V4+ this is async only