Move I18nService and and its Envelope/Channel APIs to its own module

Following the approach we did with the Guided Tour API and Keyboard Shortcuts API, we should move everything specific to i18n to its own package in the correct modules. For example: The i18nService is a class only used inside the Editor Envelope, so it should live in a module called `@kogito-tooling/i18n/dist/envelope`, for example.

Also, its Channel API (`receive_getLocale(): Promise<string>`) and its Envelope API (`
receive_localeChange(locale: string): void`) should go to `@kogito-tooling/i18n/dist/api`