ENH: Add left_semi merge

[phofl]

• [ ] closes #xxxx (Replace xxxx with the GitHub issue number)
• [x] Tests added and passed if fixing a bug or adding a new feature
• [x] All code checks passed.
• [x] Added type annotations to new arguments/methods/functions.
• [x] Added an entry in the latest doc/source/whatsnew/vX.X.X.rst file if fixing a bug or adding a new feature.

Reviving an old PR of mine

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