* 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>