feat: add DocumentType metadata support

[scolladon]

Explain your changes

---

Add DocumentType metadata support
This metadata is available since v47.
It is located in the documentTypes folder with the documentType suffix.
It does not have meta file associated

Does this close any currently open issues?

---

closes #443

[codeclimate]

Code Climate has analyzed commit 778561b and detected 0 issues on this pull request.

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (3d7960b) 100.00% compared to head (778561b) 100.00%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #720   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           35        35           
  Lines          988       988           
  Branches        97        97           
=========================================
  Hits           988       988           

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[github-actions]

Shipped in release v5.28.0.
You can install the new version using the version number or the latest-rc channel

$ sfdx plugins:install sfdx-git-delta@latest-rc
$ sfdx plugins:install [email protected]

Happy incremental deployment!