New user script example (#438)

* New user script example

This PR is the content from [PR 426](https://github.com/mdn/webextensions-examples/pull/426), originally submitted by Irene, addressing the feedback on the readme content.

* Including the following changes:
* renaming the content script example and updating the readme file to reference the user script example
* renaming the user script example and versioning as v1
* addressing feedback on the user script example: limiting input to hosts and script, addressing code and HTML/CSS comments

I haven't changed the "eating" script example, I feel it is useful to have similar demonstrations in the content and user script examples to make it easier for developers to compare and contrast.

* Various changes for feedback

* Added a user script ID that is stored in the registered script's metadata and then used in the API script to key the data stored in local storage

* Update to emphasize scoping of local storage using an ID provided in the user script metadata

Co-authored-by: Richard Bloor <rbloor@atlassian.com>

examples.json

@@ -94,6 +94,15 @@
         ],
         "name": "commands"
     },
+    {
+        "description": "Illustrates how an extension can register URL-matching content scripts at runtime.",
+        "javascript_apis": [
+            "contentScripts.register",
+            "runtime.onMessage",
+            "runtime.sendMessage"
+        ],
+        "name": "content-script-register"
+    },
     {
         "description": "Add a context menu option to links to copy the link to the clipboard, as plain text and as a link in rich HTML.",
         "javascript_apis": [
@@ -546,13 +555,13 @@
         "name": "user-agent-rewriter"
     },
     {
-        "description": "Illustrates how an extension can register URL-matching content scripts at runtime.",
+        "description": "Illustrates how an extension can register URL-matching user scripts at runtime.",
         "javascript_apis": [
-            "contentScripts.register",
+            "userScripts.register",
             "runtime.onMessage",
             "runtime.sendMessage"
         ],
-        "name": "user-script"
+        "name": "user-script-register"
     },
     {
         "description": "Demonstrates how to use webpack to package npm modules in an extension.",