chore: Add pull request template

.github/PULL_REQUEST_TEMPLATE.md

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+## Goal
+
+<!-- What is the intent of this change?
+e.g. "When initializing the Bugsnag client, it is currently difficult to (...)
+      this change simplifies the process by (...)"
+
+     "Improves the performance of data filtering"
+
+     "Adds additional test coverage to multi-threaded use of Configuration
+      objects"
+-->
+
+<!-- For new features, include design documentation:
+
+## Design
+
+Why was this approach to the goal used?
+
+-->
+
+## Changeset
+
+<!-- What structures or properties or functions were:
+
+### Added
+
+### Removed
+
+### Changed
+
+-->
+
+## Tests
+
+<!-- How was this change tested? What manual and automated tests were
+     run/added? -->
+
+## Discussion
+
+### Alternative Approaches
+
+<!-- What other approaches were considered or discussed? -->
+
+### Outstanding Questions
+
+<!-- Are there any parts of the design or the implementation which seem
+     less than ideal and that could require additional discussion?
+     List here: -->
+
+### Linked issues
+
+<!--
+
+Fixes #
+Related to #
+
+-->
+
+## Review
+
+<!-- When submitting for review, consider the points for self-review and the
+     criteria which will be used for secondary review -->
+
+For the submitter, initial self-review:
+
+- [ ] Commented on code changes inline explain the reasoning behind the approach
+- [ ] Reviewed the test cases added for completeness and possible points for discussion
+- [ ] A changelog entry was added for the goal of this pull request
+- [ ] Check the scope of the changeset - is everything in the diff required for the pull request?
+- This pull request is ready for:
+  - [ ] Initial review of the intended approach, not yet feature complete
+  - [ ] Structural review of the classes, functions, and properties modified
+  - [ ] Final review
+
+For the pull request reviewer(s), this changeset has been reviewed for:
+
+- [ ] Consistency across platforms for structures or concepts added or modified
+- [ ] Consistency between the changeset and the goal stated above
+- [ ] Internal consistency with the rest of the library - is there any overlap between existing interfaces and any which have been added?
+- [ ] Usage friction - is the proposed change in usage cumbersome or complicated?
+- [ ] Performance and complexity - are there any cases of unexpected O(n^3) when iterating, recursing, flat mapping, etc?
+- [ ] Concurrency concerns - if components are accessed asynchronously, what issues will arise
+- [ ] Thoroughness of added tests and any missing edge cases
+- [ ] Idiomatic use of the language