* removed all the generated files (<locale>.json, <locale>.js) except for ‘en’ versions and added them to git ignore.
* updated cleanup.sh to use our messages.
* added validation check to fail if any translations include newlines
* skip generating js files other than ‘en’ as we don’t use them
* Revised messages.js to use _single_ quoted strings instead of double (consistent with Blockly - if we need to import more strings), and switch the validations to check for single not double quotes.
* additional small changes for simplicity and ergonomics
Not handling category menu at this time as it’s managed differently in scratch-gui.
* Added script to create combined locales `Blockly.ScratchMsgs.locales`
* Regenerated en.json after changing single quoted strings in messages.js to double quoted strings.
* Include `create_scratch_msgs.js` in `npm run translate`
locale=<locale-string>` to the URL.
* Added assertions so translate fails if `messages.js` has incorrect strings, or if any of the downloaded locale json files have errors in the translations.
* Playground will load different locales.
