Rewrite @tolgee/core

[JanCizmar]

• [x] Make it tiny
• [x] Make everything which is not essential as an external module
• [x] Create a simple message format which would only support parameters passed in curly brackets {} and which is compatible with ICU, so users, who will add ICU module in the future wouldn't need to rewrite their messages
• [x] Support for namespaces / splitting translations into separate files
• [ ] Markdown support (as external module)
• [ ] Support for basic HTML tags (as external module)

[stepan662]

Mostly solved in https://github.com/tolgee/tolgee-js/pull/3109