Sync providers: extract into its own folder

[cuserox]

Context 📝

The implementation of sync providers in the plugin is repetitive and can be optimized. The aim of this task is to gather together as much of the UI & logic into its own realm, to set a base that allows us to publish as an independent package that can be flexible enough for future providers.

Acceptance Criteria 🛠️

• Move all related constants, methods and component into a single folder in the plugin
• Improve tests

🎯 A package that lets users create/choose a sync provider, and then have a button that Import variables which would basically just read local variables from Figma, and push those to the repository. So in effect we'd have all of these parts in the Figma plugin already, but we want those to be part of a new plugin that does just that. As in it'd be one-directional for now (pushing to git), but eventually would do both.

Available providers to extract, ordered by popularity (https://eu.mixpanel.com/project/2151665/view/306667/app/insights/?discover=1#report/27296141)

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1. Extract GitHub into /sync-providers #3112
   sync github +1
   
2. Extract GitLab into /sync-providers
3. Extract ADO into /sync-providers
4. Extract URL into sync-providers
5. Extract JSONBin into /sync-providers
6. Extract Bitbucket into /sync-providers
7. Extract Supernova into /sync-providers
8. Extract Generic Versioned Storage into /sync-providers
9. Extract Tokens Studio into /sync-providers

[cuserox]

⚠️ WIP branch: https://github.com/tokens-studio/figma-plugin/tree/feat/3000-extract-sync-providers