Bump version to 2.0.0 and add note about module compatibility

- Bump version to `2.0.0`.
- Add note about ESM compatibility in README.md file.
- Remove dollar sign used before commands without showing output, refer to https://github.com/markdownlint/markdownlint/blob/v0.13.0/docs/RULES.md#md014---dollar-signs-used-before-commands-without-showing-output

⚠ BREAKING CHANGE: Package is now ESM-only.
For CJS compatibility to switch to the `1.x` version.

README.md

@@ -11,9 +11,14 @@
 Via npm :
 
 ```bash
-$ npm i @xn-02f/gravatar
+npm i @xn-02f/gravatar
 ```
 
+> [!NOTE]
+> This library was moved to ESM from `v2`.
+>
+> If ESM doesn't work well with your node version, switch to the `1.x` version for CJS compatibility.
+
 ## Usage
 
 ```javascript

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xn-02f/gravatar",
-  "version": "1.3.3",
+  "version": "2.0.0",
   "description": "A library to generate gravatar image url.",
   "type": "module",
   "exports": "./gravatar.js",