build: run build:prod on yarn prepare

Adds `yarn build:prod` to the `yarn prepare` script.

For most package managers (e.g. yarn v1 and NPM),
the `prepare` script is called automatically when installing
from a local location/git url. It's not called when installing
from NPM.

This is required because the source code doesn't contain
the `dist/mermaid.min.js` file, it has to be built first.

Fixes installing mermaid via `git`, e.g.:
`yarn add git+https://github.com/mermaid-js/mermaid.git`

Fixes: 1549eb20dfbf0698749ab50b2cb264e63d2015b5

package.json

@@ -41,7 +41,7 @@
     "test": "yarn lint && jest src/.*",
     "test:watch": "jest --watch src",
     "prepublishOnly": "yarn build && yarn test",
-    "prepare": "husky install",
+    "prepare": "concurrently \"husky install\" \"yarn build:prod\"",
     "pre-commit": "lint-staged"
   },
   "repository": {
@@ -132,4 +132,4 @@
     "**/*.css",
     "**/*.scss"
   ]
-}
+}