Add diff-api-snapshot action to danger (#52045)

Summary:
This is a work in progress PR that adds breaking change detection to danger.



## Changelog:

<!-- Help reviewers and the release process by writing your own changelog entry.

Pick one each for the category and type tags:

[ANDROID|GENERAL|IOS|INTERNAL] [BREAKING|ADDED|CHANGED|DEPRECATED|REMOVED|FIXED|SECURITY] - Message

For more details, see:
https://reactnative.dev/contributing/changelogs-in-pull-requests


Reviewed By: huntie

Differential Revision: D76735630

Pulled By: coado

Author: coado

.github/actions/diff-js-api-breaking-changes/action.yml

@@ -0,0 +1,23 @@
+name: diff-js-api-breaking-changes
+description: Check for breaking changes in the public React Native JS API
+runs:
+  using: composite
+  steps:
+    - name: Fetch snapshot from PR head
+      shell: bash
+      env:
+        SCRATCH_DIR: ${{ runner.temp }}/diff-js-api-breaking-changes
+      run: |
+        mkdir $SCRATCH_DIR
+        git fetch --depth=1 origin ${{ github.event.pull_request.head.sha }}
+        git show ${{ github.event.pull_request.head.sha }}:packages/react-native/ReactNativeApi.d.ts > $SCRATCH_DIR/ReactNativeApi-after.d.ts \
+          || echo "" > $SCRATCH_DIR/ReactNativeApi.d.ts
+    - name: Run breaking change detection
+      shell: bash
+      env:
+        SCRATCH_DIR: ${{ runner.temp }}/diff-js-api-breaking-changes
+      run: |
+        node ./scripts/diff-api-snapshot \
+        ${{ github.workspace }}/packages/react-native/ReactNativeApi.d.ts  \
+        $SCRATCH_DIR/ReactNativeApi-after.d.ts \
+        > $SCRATCH_DIR/output.json

.github/workflows/danger-pr.yml

@@ -22,6 +22,8 @@ jobs:
         uses: ./.github/actions/setup-node
       - name: Run yarn install
         uses: ./.github/actions/yarn-install
+      - name: Run diff-js-api-breaking-changes
+        uses: ./.github/actions/diff-js-api-breaking-changes
       - name: Danger
         run: yarn danger ci --use-github-checks --failOnErrors
         working-directory: private/react-native-bots

private/react-native-bots/dangerfile.js

@@ -11,6 +11,8 @@
 'use strict';
 
 const {danger, fail, warn} = require('danger');
+const fs = require('fs');
+const path = require('path');
 
 const body = danger.github.pr.body?.toLowerCase() ?? '';
 
@@ -28,6 +30,25 @@ const isFromPhabricator = body_contains('differential revision:');
 // Provides advice if a summary section is missing, or body is too short
 const includesSummary = body_contains('## summary', 'summary:');
 
+const snapshot_output = JSON.parse(
+  fs.readFileSync(
+    path.join(
+      process.env.RUNNER_TEMP,
+      'diff-js-api-breaking-changes/output.json',
+    ),
+    'utf8',
+  ),
+);
+if (snapshot_output && snapshot_output.result !== 'NON_BREAKING') {
+  const title = ':exclamation: JavaScript API change detected';
+  const idea =
+    'This PR commits an update to ReactNativeApi.d.ts, indicating a change to React Native's public JavaScript API. ' +
+    'Please include a clear changelog message. ' +
+    'This change will be subject to extra review.\n\n' +
+    `This change was flagged as: <code>${snapshot_output.result}</code>`;
+  warn(`${title} - <i>${idea}</i>`);
+}
+
 const hasNoUsefulBody =
   !danger.github.pr.body || danger.github.pr.body.length < 50;
 const hasTooShortAHumanSummary =