chore(repo): Add yarn commit for structured commit messages (#4660)

**Summary**

Adds commitizen and adopts the cz-conventional-changelog adapter for commit logs. Will be followed
up by commit message linters and automatic version number bumping in CI builds.

**Test plan**

Run `yarn install` and then `yarn commit -a` after making some changes to test the new awesomeness. This commit message was also generated via that command.

Author: BYK

README.md

@@ -13,8 +13,8 @@
   <a href="https://circleci.com/gh/yarnpkg/yarn"><img alt="Circle Status" src="https://circleci.com/gh/yarnpkg/yarn.svg?style=shield&circle-token=5f0a78473b0f440afb218bf2b82323cc6b3cb43f"></a>
   <a href="https://ci.appveyor.com/project/kittens/yarn/branch/master"><img alt="Appveyor Status" src="https://ci.appveyor.com/api/projects/status/0xdv8chwe2kmk463?svg=true"></a>
   <a href="https://discord.gg/yarnpkg"><img alt="Discord Chat" src="https://img.shields.io/discord/226791405589233664.svg"></a>
+  <a href="http://commitizen.github.io/cz-cli/"><img alt="Commitizen friendly" src="https://img.shields.io/badge/commitizen-friendly-brightgreen.svg"></a>
 </p>
-
 ---
 
 **Fast:** Yarn caches every package it has downloaded, so it never needs to download the same package again. It also does almost everything concurrently to maximize resource utilization. This means even faster installs.

package.json

@@ -61,6 +61,8 @@
     "babel-preset-node5": "^10.2.0",
     "babel-preset-stage-0": "^6.0.0",
     "babylon": "^6.5.0",
+    "commitizen": "^2.9.6",
+    "cz-conventional-changelog": "^2.0.0",
     "eslint": "^4.3.0",
     "eslint-config-fb-strict": "^20.1.0-delta.3",
     "eslint-plugin-babel": "^4.0.0",
@@ -111,16 +113,26 @@
     "test-ci": "yarn build && yarn test-only",
     "test-only": "node --max_old_space_size=4096 node_modules/jest/bin/jest.js --verbose",
     "test-coverage": "node --max_old_space_size=4096 node_modules/jest/bin/jest.js --coverage --verbose",
-    "watch": "gulp watch"
+    "watch": "gulp watch",
+    "commit": "git-cz"
   },
   "jest": {
-    "collectCoverageFrom": ["src/**/*.js"],
+    "collectCoverageFrom": [
+      "src/**/*.js"
+    ],
     "testEnvironment": "node",
-    "modulePathIgnorePatterns": ["__tests__/fixtures/"],
+    "modulePathIgnorePatterns": [
+      "__tests__/fixtures/"
+    ],
     "testPathIgnorePatterns": [
       "__tests__/(fixtures|__mocks__)/",
       "updates/",
       "/_(temp|mock|install|init|helpers).js$"
     ]
+  },
+  "config": {
+    "commitizen": {
+      "path": "./node_modules/cz-conventional-changelog"
+    }
   }
 }