add missing safestring methods

types/nunjucks/index.d.ts

@@ -140,6 +140,10 @@ export class PrecompiledLoader extends Loader implements ILoader {
 
 export namespace runtime {
     class SafeString {
-        constructor(...args: any[]);
+        constructor(val: string);
+        val: string;
+        length: number;
+        valueOf(): string;
+        toString(): string;
     }
 }

types/nunjucks/nunjucks-tests.ts

@@ -53,4 +53,4 @@ class MyLoader extends nunjucks.Loader implements nunjucks.ILoader {
 
 env = new nunjucks.Environment(new MyLoader());
 
-new nunjucks.runtime.SafeString();
+new nunjucks.runtime.SafeString("an unsafe string");