build: target ES2019 in TypeScript

Author: aminya

test/tsconfig.json

@@ -2,7 +2,12 @@
   "extends": "../tsconfig.json",
   "include": ["./unit/**/*.ts", "./*.ts"],
   "compilerOptions": {
-    "skipLibCheck": true
+    "skipLibCheck": true,
+    "lib": [
+      "ES2019",
+      // only used in tests
+      "ES2021.WeakRef"
+    ]
   },
   "compileOnSave": false
 }

tsconfig.json

@@ -2,7 +2,7 @@
   "root": true,
   "compilerOptions": {
     "allowJs": false,
-    "target": "es2020",
+    "target": "ES2019",
     "declaration": true,
     "module": "commonjs",
     "moduleResolution": "node",
@@ -19,11 +19,7 @@
     "incremental": true,
     "sourceMap": true,
     "esModuleInterop": true,
-    "lib": [
-      "ES2020",
-      // only used in tests
-      "ES2021.WeakRef"
-    ]
+    "lib": ["ES2019"]
   },
   "compileOnSave": true
 }