As of #23867 this is now a real package, so updating the imports to
use the real package name, rather than relying on the alias. The
name change in the package name is because `I18n` is not a valid
name as NPM packages must be all lowercase.
This commit also introduces an eslint rule to prevent importing from
the old I18n path.
For themes/plugins, the old 'i18n' name remains functional.
When editing the files for a theme in the admin dashboard, typing "cmd+s" (a common key-binding to save in most text editors) used to engage the browser's default "save page" dialogue.
This commit adds a key-binding to the ace editor that saves the file.
Now, the "cmd+s" (and "ctrl+s" for windows) key-binding does the same action as the save button.
