Fix missing not nil constraint for pairs keys

.gitignore

@@ -1,3 +1,4 @@
 node_modules
 .vscode
 *.log
+*.js

core/global.d.ts

@@ -164,7 +164,7 @@ declare function next(table: object, index?: any): LuaMultiReturn<[any, any] | [
  * See function next for the caveats of modifying the table during its
  * traversal.
  */
-declare function pairs<TKey, TValue>(
+declare function pairs<TKey extends AnyNotNil, TValue>(
     t: LuaTable<TKey, TValue>
 ): LuaIterable<LuaMultiReturn<[TKey, NonNullable<TValue>]>>;
 declare function pairs<T>(t: T): LuaIterable<LuaMultiReturn<[keyof T, NonNullable<T[keyof T]>]>>;

test/testproject/main.ts

@@ -0,0 +1,5 @@
+/// <reference path="../../5.4.d.ts" />
+const mytable = new LuaMap<string, number>();
+
+for (const [k, v] of pairs(mytable)) {
+}

test/testproject/package.json

@@ -0,0 +1,6 @@
+{
+    "devDependencies": {
+        "typescript": "~4.8.2",
+        "typescript-to-lua": "~1.9.0"
+    }
+}

test/testproject/tsconfig.json

@@ -0,0 +1,7 @@
+{
+    "compilerOptions": {
+        "strict": true,
+        "target": "esnext",
+        "types": ["typescript-to-lua/language-extensions"]
+    }
+}

test/tsconfig.json

@@ -1,6 +1,7 @@
 {
     "compilerOptions": {
+        "rootDir": ".",
         "types": ["node", "jest"]
     },
-    "include": ["."]
+    "include": ["./*.ts"]
 }