Update Pull Request template (#59062)

# Objective

Give a bit more structure to contributors so that each (potentially new)
contributor doesn't have to come up with their own structure on-the-fly.

## Solution

Update the PR template to include sections for describing

- what the PR is trying to achieve
- how it decided to achieve the goal
- how reviewers can test the work
- demos of the new functionality

## Testing

I used the PR template for this PR, so reviewers can evaluate if they
think the structure is useful.

## Self-Review Checklist:

- [x] I've reviewed my own diff for quality, security, and reliability
- [x] Unsafe blocks (if any) have justifying comments
- [x] The content adheres to Zed's UI standards
([UX/UI](https://github.com/zed-industries/zed/blob/main/CONTRIBUTING.md#uiux-checklist)
and
[icon](https://github.com/zed-industries/zed/blob/main/crates/icons/README.md)
guidelines)
- [x] Tests cover the new/changed behavior
- [x] Performance impact has been considered and is acceptable

## Showcase

<details>
  <summary>Click to view showcase</summary>

### Before

<img width="1082" height="572" alt="image"
src="https://github.com/user-attachments/assets/bc72b8ab-f5ec-4270-b1ec-546456525e92"
/>

### After

<img width="1241" height="945" alt="image"
src="https://github.com/user-attachments/assets/088d90e4-2cb1-42cd-9f8f-0b2783ab7fe4"
/>


</details>

---

Release Notes:

- N/A

.github/pull_request_template.md

@@ -1,4 +1,20 @@
-Self-Review Checklist:
+# Objective
+
+- Describe the objective or issue this PR addresses.
+- If you're fixing a specific issue, use "Fixes #X" for each issue as [described in the GitHub docs](https://docs.github.com/en/issues/tracking-your-work-with-issues/using-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword).
+
+## Solution
+
+- Describe the solution used to achieve the objective above.
+
+## Testing
+
+- Did you test these changes? If so, how?
+- Are there any parts that need more testing?
+- How can other people (reviewers) test your changes? Is there anything specific they need to know?
+- If relevant, what platforms did you test these changes on, and are there any important ones you can't test?
+
+## Self-Review Checklist:
 
 - [ ] I've reviewed my own diff for quality, security, and reliability
 - [ ] Unsafe blocks (if any) have justifying comments
@@ -6,7 +22,24 @@ Self-Review Checklist:
 - [ ] Tests cover the new/changed behavior
 - [ ] Performance impact has been considered and is acceptable
 
-Closes #ISSUE
+## Showcase
+
+> This section is optional. If this PR does not include a visual change or does not add a new user-facing feature, you can delete this section.
+
+- Help others understand the result of this PR by showcasing your awesome work!
+- If this PR includes a visual change, consider adding a screenshot, GIF, or video
+  - A before/after comparison is very useful for changes to existing features!
+
+While a showcase should aim to be brief and digestible, you can use a toggleable section to save space on longer showcases:
+
+<details>
+  <summary>Click to view showcase</summary>
+
+My super cool demos here
+
+</details>
+
+---
 
 Release Notes: