Add PR templates

This adds PR templates for the different classes of changes we support,
according to VERSIONING.md.

Author: DirectXMan12

.github/PULL_REQUEST_TEMPLATE/breaking_change.md

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+---
+name: Breaking change (X)
+about: A breaking change to the API surface or expected behavior of this project. 
+
+---
+
+<!-- please add a :warning: (`:warning:`) to the title of this PR, and delete this line and similar ones -->
+
+<!-- What does this do, and why do we need it? -->
+
+<!-- Why does this have to be a breaking change (what else did you consider)? -->

.github/PULL_REQUEST_TEMPLATE/bug_fix.md

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+---
+name: Bug fix (Z)
+about: A bug fix that doesn't otherwise change API surface
+
+---
+
+<!-- please add a :bug: (`:bug:`) to the title of this PR, and delete this line and similar ones -->
+
+<!-- What does this do, and why do we need it? -->
+
+<!-- What issue does this fix (e.g. Fixes #XYZ) -->

.github/PULL_REQUEST_TEMPLATE/compat_feature.md

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+---
+name: Backwards-compatible feature (Y)
+about: A new feature that adds to the API surface or behavior, but doesn't break backwards compatibility
+
+---
+
+<!-- please add a :sparkles: (`:sparkles:`) to the title of this PR, and delete this line and similar ones -->
+
+<!-- What does this do, and why do we need it? -->

.github/PULL_REQUEST_TEMPLATE/docs.md

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+---
+name: Documentation
+about: A change to the documentation of this repository
+
+---
+
+<!-- please add a :book: (`:book:`) to the title of this PR, and delete this line and similar ones -->
+
+<!-- What docs does this change, and why? -->

.github/PULL_REQUEST_TEMPLATE/other.md

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+---
+name: Tests/Infra/Other
+about: A change to the tests in this repo, the utilities scripts, or some other piece of infrastructure tooling
+
+---
+
+<!-- please add a :running: (`:running:`) to the title of this PR, and delete this line and similar ones -->
+
+<!-- What does this do, and why do we need it? -->