deps: Redo ESLint upgrade to ^6.8.0, this time adding core-js.

A redo of 01593b3, after we reverted it in e3e8d9a.

One thing we realized [1] is that we don't have to wait for
`prettier-eslint` to fix the problem of its missing `core-js`
dependency; we can provide it ourselves. We do so by just adding it
directly to our own `dependencies`.

So, in addition to the commands listed in 01593b3, also run
`yarn add core-js@^3.1.4`. We choose ^3.1.4 because that's what was
there before prettier/prettier-eslint#338. The open PR I mention in
e3e8d9a (prettier/prettier-eslint#359) suggests ^3.6.5, but it
seems to do so at random; might as well approach as closely as
possible a configuration that we know works.

Now, we have the ESLint upgrade, and format-on-save works as
expected.

[1] https://chat.zulip.org/#narrow/stream/243-mobile-team/topic/SOLVED.3A.20Prettier.20auto-formatting/near/982426

Fixes: #4120

package.json

@@ -41,7 +41,9 @@
     "base-64": "^0.1.0",
     "blueimp-md5": "^2.10.0",
     "color": "^3.0.0",
+    "core-js": "^3.1.4",
     "date-fns": "^1.29.0",
+    "eslint-plugin-react-hooks": "^4.0.8",
     "expo-apple-authentication": "^2.1.1",
     "expo-application": "^2.1.1",
     "expo-screen-orientation": "^1.0.0",
@@ -98,15 +100,15 @@
     "babel-eslint": "^10.0.3",
     "babel-plugin-transform-remove-console": "^6.9.4",
     "deep-freeze": "^0.0.1",
-    "eslint": "^5.16.0",
-    "eslint-config-airbnb": "^17.1.1",
-    "eslint-config-prettier": "^6.0.0",
-    "eslint-plugin-flowtype": "^3.9.0",
-    "eslint-plugin-import": "^2.18.1",
-    "eslint-plugin-jest": "^22.11.1",
-    "eslint-plugin-jsx-a11y": "^6.2.3",
-    "eslint-plugin-prettier": "^3.1.0",
-    "eslint-plugin-react": "^7.14.2",
+    "eslint": "^6.8.0",
+    "eslint-config-airbnb": "^18.2.0",
+    "eslint-config-prettier": "^6.11.0",
+    "eslint-plugin-flowtype": "^4.7.0",
+    "eslint-plugin-import": "^2.22.0",
+    "eslint-plugin-jest": "^22.21.0",
+    "eslint-plugin-jsx-a11y": "^6.3.1",
+    "eslint-plugin-prettier": "^3.1.4",
+    "eslint-plugin-react": "^7.20.5",
     "flow-bin": "^0.105.0",
     "flow-coverage-report": "^0.6.0",
     "flow-typed": "^2.4.0",
@@ -120,7 +122,7 @@
     "lolex": "^5.1.1",
     "metro-react-native-babel-preset": "^0.56.0",
     "prettier": "^1.18.2",
-    "prettier-eslint": "^9.0.0",
+    "prettier-eslint": "^11.0.0",
     "prettier-eslint-cli": "^5.0.0",
     "prop-types": "^15.7.2",
     "react-dom": "16.9.0",