typescript: Fix errors hidden by skipLibCheck.

This requires temporarily downgrading to @types/node@^12 (see
https://github.com/electron/electron/issues/21612).

Leave skipLibCheck on for now as it still saves a few seconds when
running tsc.

Signed-off-by: Anders Kaseorg <anders@zulipchat.com>

package-lock.json

@@ -697,9 +697,9 @@
       "dev": true
     },
     "@types/node": {
-      "version": "13.9.3",
-      "resolved": "https://registry.npmjs.org/@types/node/-/node-13.9.3.tgz",
-      "integrity": "sha512-01s+ac4qerwd6RHD+mVbOEsraDHSgUaefQlEdBbUolnQFjKwCr7luvAlEwW1RFojh67u0z4OUTjPn9LEl4zIkA=="
+      "version": "12.12.31",
+      "resolved": "https://registry.npmjs.org/@types/node/-/node-12.12.31.tgz",
+      "integrity": "sha512-T+wnJno8uh27G9c+1T+a1/WYCHzLeDqtsGJkoEdSp2X8RTh3oOCZQcUnjAx90CS8cmmADX51O0FI/tu9s0yssg=="
     },
     "@types/normalize-package-data": {
       "version": "2.4.0",

package.json

@@ -183,6 +183,7 @@
     "@types/escape-html": "0.0.20",
     "@types/fs-extra": "^8.1.0",
     "@types/i18n": "^0.8.6",
+    "@types/node": "^12.12.31",
     "@types/request": "^2.48.4",
     "@types/requestidlecallback": "^0.3.1",
     "@typescript-eslint/eslint-plugin": "^2.25.0",
@@ -248,12 +249,19 @@
           "scripts/notarize.js",
           "tests/**/*.js",
           "tools/locale-helper/index.js",
-          "tools/reinstall-node-modules.js",
-          "typings.d.ts"
+          "tools/reinstall-node-modules.js"
         ],
         "parserOptions": {
           "sourceType": "script"
         }
+      },
+      {
+        "files": [
+          "**/*.d.ts"
+        ],
+        "rules": {
+          "import/unambiguous": "off"
+        }
       }
     ]
   }

typings.d.ts

@@ -1,4 +1,3 @@
-'use strict';
 declare module '@electron-elements/send-feedback';
 declare module 'node-mac-notifier';
 declare module 'wurl';