Enable incremental typescript builds (#4092)

FEATURE
INTERNAL
Enable incremental builds in the root tsconfig.json to increase build speed during development. For example, running `yarn build` in tfjs-backend-cpu takes ~43 seconds without incremental builds and ~30 seconds with incremental builds (after the first build).

Co-authored-by: Ping Yu <[email protected]>

Author: mattsoulanille

tsconfig.json

@@ -21,7 +21,8 @@
     "noUnusedParameters": false,
     "pretty": true,
     "noFallthroughCasesInSwitch": true,
-    "allowUnreachableCode": false
+    "allowUnreachableCode": false,
+    "incremental": true
   },
   "include": [
     "src/"