Move loading of en locale into Component.js files, use __locales__ for Locale messages, and update tests.

[dpvc]

This PR moves the loading of the en locale from the component definitions (using the en.js files in components/mjs/...) to their __locales__/Component.ts files instead. That means that those building MathJax apps using direct imports of the MathJax modules will get the English error messages loaded automatically, and we don't have to have a message about not having called Locale.setLocale(). This means the localization isn't a breaking change.

A second change is that the localization of the messages in the Locale object itself are now in a new ts/util/Locale/__locales__ directory. This means we don't have to have all the languages' messages in the Locale object initially, and since we now load the en messages automatically, that has the same effect as having them in line. The locale fake component is the first one registered, so it will always be loaded first when the locale is changed, so locale messages should be available if they are needed once a locale is changed (if not, the fallback to English will at least give a proper, if unlocalized, message). A copy command for the utility locales is added to package.json to accommodate this, and a new components/mjs/locale component is created that only copies the files (like the components/mjs/sre component does) is added.

The Locale test file is updated to get coverage for some changes in Locale.ts and the clean up registered components or messages after tests that set them.

The ts/util/Locale.ts file was modified to define a COMPONENT like the ones from __locales__/Component.ts, but we can't use Component.ts file for Locale because the Locale object has to be defined before we can call Locale.registerLocaleFiles(), like we do on the other Component.ts files. Instead, we import the en.json file directly and register it in this file after Locale is defined. We also change registerLocaleFiles() to avoid overwriting a previous registration if there was already one made earlier (e.g., if the ui/menu and a11y/explorer components are loaded separately, both of which register the a11y/aria fake component).

The registerMessages() method now produces an error message if you try to register messages for a component that isn't registered (addressing your review comment in #1494.

Finally, I shortened the localization keys, and added the new message to the en.json and de.json files.

I know this looks like a lot of files, but most of them are just deleting the en.js files from components/mjs/..., removing the imports of those files from the component definitions, and adding the loading and registering of en to all the __locales__/Component.ts files.