Allow entrytype as sorting criterion

[Siedlerchr]

Fixes https://discourse.jabref.org/t/export-sort-by-entry-type/2122/1

Entrytype was missing in sort criterion.
I did not add it to the common fields, because the common fields are also used for the customize entrytype

• Change in CHANGELOG.md described (if applicable)
• Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
• Screenshots added in PR description (for UI changes)
• Checked documentation: Is the information available and up to date? If not created an issue at https://github.com/JabRef/user-documentation/issues or, even better, submitted a pull request to the documentation repository.