feat: Adds descriptive warning for undefined x-for key and updates tests.

[nickdawes]

Description:
This pull request addresses the issue where the x-for directive would throw a generic error message when the provided key is undefined or invalid.

The changes include:
Updating the Alpine.js code to throw a more descriptive error message when the key is undefined or invalid in the x-for directive.
Adding a new test case to the Cypress test suite to check for the updated error message when the key is undefined.

Here is the summary of the changes made:
Updated the error message for an undefined or invalid key in the x-for directive.
Added a new test case to the Cypress test suite to check for the updated error message.

Why was this change necessary?
By providing a more descriptive error message, developers can quickly identify the issue and fix their code accordingly.

Please review the changes and let me know what you think.

[calebporzio]

Thanks!

[ekwoka]

Is this the correct place for this message to appear? Couldn't it be checking the result of the key evaluation?