feat: new by_cases! tactic

[JovanGerb]

This PR defines the by_cases! h : p tactic, which is a macro for by_cases h : p followed by a try push_neg at h in the second goal. We use try push_neg instead of push_neg to make the tactic also succeed when no negation is pushed. Should there maybe be an optional linter telling you to use by_cases instead if possible?

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[github-actions]

PR summary dde1cfffb6

Import changes for modified files

Dependency changes

File	Base Count	Head Count	Change
Mathlib.Tactic	2877	2879	+2 (+0.07%)

Import changes for all files

Files	Import difference
Mathlib.Tactic	2
Mathlib.Tactic.ByCases (new file)	77

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Declarations diff

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