Setup Danger (zed-industries#6994)

This PR sets up [Danger](https://danger.systems/js/) to help us codify
some of our PR rules.

As an initial rule, Danger will check to ensure that every PR has a
`Release Notes` section:

<img width="943" alt="Screenshot 2024-01-29 at 11 50 12 AM"
src="https://github.com/zed-industries/zed/assets/1486634/4d56e759-e72f-4bc0-8e74-42c55e2e6888">

Release Notes:

- N/A

.github/workflows/danger.yml

@@ -0,0 +1,35 @@
+name: Danger
+
+on:
+  pull_request:
+    branches: [main]
+    types:
+      - opened
+      - synchronize
+      - reopened
+      - edited
+
+jobs:
+  danger:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v4
+
+      - uses: pnpm/action-setup@v2.2.4
+        with:
+          version: 8
+
+      - name: Setup Node
+        uses: actions/setup-node@v4
+        with:
+          node-version: "20"
+          cache: "pnpm"
+          cache-dependency-path: "script/danger/pnpm-lock.yaml"
+
+      - run: pnpm install --dir script/danger
+
+      - name: Run Danger
+        run: pnpm run --dir script/danger danger ci
+        env:
+          GITHUB_TOKEN: ${{ github.token }}

script/danger/.gitignore

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+node_modules/

script/danger/dangerfile.ts

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+import { danger, warn } from "danger";
+
+const RELEASE_NOTES_PATTERN = new RegExp("Release Notes:\\r?\\n\\s+-", "gm");
+
+const hasReleaseNotes = RELEASE_NOTES_PATTERN.test(danger.github.pr.body);
+if (!hasReleaseNotes) {
+  warn(
+    [
+      "This PR is missing release notes.",
+      "",
+      'Please add a "Release Notes" section that describes the change:',
+      "",
+      "```",
+      "Release Notes:",
+      "",
+      "- (Added|Fixed|Improved) ... ([#<public_issue_number_if_exists>](https://github.com/zed-industries/zed/issues/<public_issue_number_if_exists>)).",
+      "```",
+      "",
+      'If your change is not user-facing, you can use "N/A" for the entry:',
+      "```",
+      "Release Notes:",
+      "",
+      "- N/A",
+      "```",
+    ].join("\n"),
+  );
+}

script/danger/package.json

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+{
+  "name": "danger",
+  "version": "1.0.0",
+  "private": true,
+  "main": "index.js",
+  "scripts": {
+    "danger": "danger"
+  },
+  "devDependencies": {
+    "danger": "11.3.1"
+  }
+}