chore: pin ts version (#3411)

The TypeScript project [release breaking changes as minor versions](microsoft/TypeScript#41563 (comment))
so pin to `4.0.x` until we can do the refactoring needed to upgrade
to `4.1.x`.

Author: achingbrain

package.json

@@ -85,7 +85,7 @@
     "it-concat": "^1.0.1",
     "nock": "^13.0.2",
     "rimraf": "^3.0.2",
-    "typescript": "^4.0.3"
+    "typescript": "4.0.x"
   },
   "engines": {
     "node": ">=10.3.0",