fix: add check and trigger error if autoload is not found

[ryanwelcher]

Description

If the plugin is activated before composer install is run, it will fire an error. This PR adds a check and triggers a warning to the user.

Related Issue

Closes #18

Testing Instructions

1. Remove the vendor dir on main branch and activate the plugin.
2. Confirm there is a fatal error
3. Switch to this branch and try to activate
4. Confirm that WordPress still loads but a warning is fired telling the user to run composer install

Checklist

• Have you added a note to the changelog?
• Have you ensured the code passes all tests using the included plugin-check?
• Have you checked for code style and linting errors?

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[troychaplin]

Never considered this as I always had a vendor folder. I think it might be best to commit the vendor folder and avoid this setup altogether.