Fix #13269 : support multi-file import across different formats

[070jhz]

Closes #13269

this PR improves the import command to properly handle importing multiple files with different supported formats in a single operation.

ImportCommand::execute now calls dialogService.showFileOpenDialogAndGetMultipleFiles instead of dialogService.showFileOpenDialog and importSingleFile() has been refactored to importMultipleFiles() to handle one or more files

Steps to test

1. File menu > Import > Import into current/new library
2. Select one or more files
3. ???
4. Profit

Mandatory checks

• [x] I own the copyright of the code submitted and I license it under the MIT license
• [/] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
• [ ] Tests created for changes (if applicable)
• [x] Manually tested changed features in running JabRef (always required)
• [/] Screenshots added in PR description (if change is visible to the user)
• [x] Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• [x] Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.

[Siedlerchr]

PLease add a changelog entry, otherwise it's good to go!

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[trag-bot[bot]]

@trag-bot didn't find any issues in the code! ✅✨