feat: use downlevel-dts to generate TS 3.4 compatible types (#266)

Author: trivikr

smithy-typescript-codegen/src/main/resources/software/amazon/smithy/typescript/codegen/base-package.json

@@ -13,7 +13,8 @@
     "test": "yarn build && jest --coverage --passWithNoTests",
     "build:cjs": "tsc -p tsconfig.json",
     "build:es": "tsc -p tsconfig.es.json",
-    "build": "yarn build:cjs && yarn build:es"
+    "build": "yarn build:cjs && yarn build:es",
+    "postbuild": "downlevel-dts types types/ts3.4"
   },
   "main": "./dist/cjs/index.js",
   "types": "./types/index.d.ts",
@@ -29,12 +30,18 @@
     "tslib": "^2.0.0"
   },
   "devDependencies": {
+    "downlevel-dts": "0.7.0",
     "jest": "^26.1.0",
     "rimraf": "^3.0.0",
     "typedoc": "^0.19.2",
     "typescript": "~4.1.2"
   },
   "engines": {
     "node": ">=10.0.0"
+  },
+  "typesVersions": {
+    "<3.9": {
+      "types/*": ["types/ts3.4/*"]
+    }
   }
 }