init: create the @mediamonks/prettier-config repository

* add `main.json` containing the prettier configuration
* add a `README.md` describing how to install, use and extend the
  configuration

README.md

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+# MediaMonks - Prettier Configuration
+
+The official Media.Monks prettier configuration, based on the [Frontend Coding Standards](https://github.com/mediamonks/frontend-coding-standards).
+
+## Installation
+
+The configuration can be installed via `npm`.
+
+```bash
+npm install --dev @mediamonks/prettier-config
+```
+
+To inform prettier of this configuration, you have to add the `prettier` property to your `package.json` file:
+
+```json
+"prettier": "@mediamonks/prettier-config"
+```
+
+Instead of manually editing your `package.json`, you can also utilize the `npm pkg` subcommand:
+
+```bash
+npm pkg set prettier=@mediamonks/prettier-config
+```
+
+## Extending
+
+[Prettier does not ship with a built-in way of extending configurations](https://prettier.io/docs/en/configuration.html#sharing-configurations).
+
+To extend the configuration, you will have to create a `.prettierrc.js` file (or `.prettierrc.cjs` if your package is a `"type": "module"`) and import the Media.Monks configuration using `require`:
+
+```js
+module.exports = {
+  ...require("@mediamonks/prettier-config"),
+  semi: false,
+};
+```
+
+If you have previously added the configuration to your `package.json`, via the `prettier` property, you can now remove it.
+You can also utilize the `npm pkg`:
+
+```bash
+npm pkg delete prettier
+```

main.json

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+{
+  "$schema": "http://json.schemastore.org/prettierrc",
+  "printWidth": 100,
+  "tabWidth": 2,
+  "singleQuote": true,
+  "trailingComma": "all",
+  "proseWrap": "always"
+}

package.json

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+{
+  "name": "@mediamonks/prettier-config",
+  "version": "1.0.0",
+  "description": "Default Media.Monks Prettier Configuration",
+  "keywords": [
+    "Media.Monks",
+    "prettier"
+  ],
+  "homepage": "https://github.com/mediamonks/prettier-config#readme",
+  "bugs": {
+    "url": "https://github.com/mediamonks/prettier-config/issues"
+  },
+  "repository": {
+    "type": "git",
+    "url": "git+https://github.com/mediamonks/prettier-config.git"
+  },
+  "license": "MIT",
+  "author": "Media.Monks",
+  "main": "main.json"
+}