RFR: Create pull request template

[Jorn Vernee]

Create a pull request template. The text of the template will appear as the pull request description when creating a new pull request, and contains basic instructions on how to fill out the PR description, and how to test.

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Commit messages:
 - Create PR template

Changes: https://git.openjdk.org/jextract/pull/233/files
  Webrev: https://webrevs.openjdk.org/?repo=jextract&pr=233&range=00
  Stats: 18 lines in 1 file changed: 18 ins; 0 del; 0 mod
  Patch: https://git.openjdk.org/jextract/pull/233.diff
  Fetch: git fetch https://git.openjdk.org/jextract.git pull/233/head:pull/233

PR: https://git.openjdk.org/jextract/pull/233