feat: enhance typescript definition for knownHelpers

- Add support for custom helpers while keeping
  the list of builtin-helpers to maintain for
  autocompletion support

closes #1544

types/index.d.ts

@@ -173,16 +173,7 @@ type RuntimeOptions = Handlebars.RuntimeOptions;
 interface CompileOptions {
   data?: boolean;
   compat?: boolean;
-  knownHelpers?: {
-      helperMissing?: boolean;
-      blockHelperMissing?: boolean;
-      each?: boolean;
-      if?: boolean;
-      unless?: boolean;
-      with?: boolean;
-      log?: boolean;
-      lookup?: boolean;
-  };
+  knownHelpers?: KnownHelpers;
   knownHelpersOnly?: boolean;
   noEscape?: boolean;
   strict?: boolean;
@@ -192,6 +183,22 @@ interface CompileOptions {
   explicitPartialContext?: boolean;
 }
 
+type KnownHelpers = {
+  [name in BuiltinHelperName | CustomHelperName]: boolean;
+};
+
+type BuiltinHelperName =
+  "helperMissing"|
+  "blockHelperMissing"|
+  "each"|
+  "if"|
+  "unless"|
+  "with"|
+  "log"|
+  "lookup";
+
+type CustomHelperName = string;
+
 interface PrecompileOptions extends CompileOptions {
   srcName?: string;
   destName?: string;

types/test.ts

@@ -76,7 +76,7 @@ Handlebars.registerHelper('list', (context, options: Handlebars.HelperOptions) =
   }
   return ret + "</ul>";
 });
-template6([{url:"", title:""}])
+template6([{url:"", title:""}]);
 
 
 const escapedExpression = Handlebars.Utils.escapeExpression('<script>alert(\'xss\');</script>');
@@ -88,4 +88,12 @@ const parsedTmpl = Handlebars.parse('<p>Hello, my name is {{name}}.</p>', {
   ignoreStandalone: true
 });
 
-const parsedTmplWithoutOptions = Handlebars.parse('<p>Hello, my name is {{name}}.</p>');
+const parsedTmplWithoutOptions = Handlebars.parse('<p>Hello, my name is {{name}}.</p>');
+
+// #1544, allow custom helpers in knownHelpers
+Handlebars.compile('test', {
+  knownHelpers: {
+    each: true,
+    customHelper: true
+  }
+});