chore: update typescript to 3.9.5 (#809)

slight build speed improvement, see #809.
close #809

package.json

@@ -104,7 +104,7 @@
     "tslint-loader": "^3.5.4",
     "svg2pdf.js": "^1.5.0",
     "typedoc": "^0.17.3",
-    "typescript": "^3.7.4",
+    "typescript": "^3.9.5",
     "webpack": "^4.43.0",
     "webpack-cli": "^3.3.11",
     "webpack-dev-server": "^3.10.3",