feat: added excel backend

[PeterStaar-IBM]

Adding a backend to convert excel workbooks.

Resolves #258.

Checklist:

• [x] Commit Message Formatting: Commit titles and messages follow guidelines in the conventional commits.
• [x] Documentation has been updated, if necessary. -> not necessary
• [x] Examples have been added, if necessary. -> not necessary
• [x] Tests have been added, if necessary.

[mergify[bot]]

Merge Protections

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🟢 Enforce conventional commit

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Make sure that we follow https://www.conventionalcommits.org/en/v1.0.0/

• [X] title ~= ^(fix|feat|docs|style|refactor|perf|test|build|ci|chore|revert)(?:\(.+\))?:

🟢 Require two reviewer for test updates

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When test data is updated, we require two reviewers

• [X] #approved-reviews-by >= 2