[Penpot] Import tokens

Currently in Penpot we have the option to create tokens and export them in json format. Another feature which is important for interoperability between files and tools is the ability to import or load tokens which has been worked on in the Penpot app or other tools.

Desired solution:
- Ability to import tokens
- Maintain the structure of the token sets, themes etc using the $metadata and $themes
- Ability to upload tokens in a multi-file structure
- Ability to upload tokens in both DTCG format as well as legacy token format

Current solution:
- Ability to upload tokens in single file structure
- Ability to upload tokens without set structure