chore(Bicategory): restructure oplax modifications

[callesonne]

This PR moves Oplax modifications to its own file Modification/Oplax.lean and also moves FunctorBicategory.lean to FunctorBicategory/Oplax.lean. This will improve the file structure for later PRs generalizing these constructions to pseudofunctors.

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[github-actions[bot]]

PR summary 4e5e9467f7

Import changes for modified files

No significant changes to the import graph

Import changes for all files

Files	Import difference
Mathlib.CategoryTheory.Bicategory.FunctorBicategory	-325
Mathlib.CategoryTheory.Bicategory.Modification.Oplax	325
Mathlib.CategoryTheory.Bicategory.FunctorBicategory.Oplax	326

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Declarations diff

No declarations were harmed in the making of this PR! 🐙

You can run this locally as follows

## summary with just the declaration names:
./scripts/declarations_diff.sh <optional_commit>

## more verbose report:
./scripts/declarations_diff.sh long <optional_commit>

The doc-module for script/declarations_diff.sh contains some details about this script.

[joelriou]

Thanks!

bors merge

[mathlib-bors[bot]]

Pull request successfully merged into master.

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