[High Priority - 1] Add Absolute Path API + Implement CurDir

[satviksr]

Fixes #38, #36, #15, #37

• [x] Implements absolute path API
• [x] Migrate Current Directory API calls
• [x] Tests absolute path API
• [x] Tests Regex Expressions for validations
• [x] Implements CurDir
• [x] Tests for CurDir

Please review now but merge this after #22 is merged.

Update: I plan to differ migration current directory calls as we need to migrate to the new FileMapper + Current Directory calls. It is difficult as there are pending PRs to merge. (Trying to avoid conflicts)

Update 2: I migrated most of the calls now as FileMapper is merged.

[todo[bot]]

Implement relative path validation (regex)

Currently only absolute paths are being validated

---

https://github.com/googleinterns/filesystem-in-appsscript/blob/db56c86390fea0a9380060b33cfceaf7a1e3a117/FileSystem/DirectoryManager.js#L55-L60

---

This comment was generated by todo based on a todo comment in db56c86390fea0a9380060b33cfceaf7a1e3a117 in #28. cc @googleinterns.