[ci] Automatically label PRs from core team (#32100)

Adds a new `MAINTAINERS` file which contains github usernames of core
team members. This file serves as documentation for core team membership
and is also used to automatically label PRs from core.
---
[//]: # (BEGIN SAPLING FOOTER)
Stack created with [Sapling](https://sapling-scm.com). Best reviewed
with [ReviewStack](https://reviewstack.dev/facebook/react/pull/32100).
* #32101
* __->__ #32100

.github/workflows/shared_core_label.yml

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+name: (Shared) Core Label
+
+on:
+  pull_request_target:
+
+env:
+  TZ: /usr/share/zoneinfo/America/Los_Angeles
+  # https://github.com/actions/cache/blob/main/tips-and-workarounds.md#cache-segment-restore-timeout
+  SEGMENT_DOWNLOAD_TIMEOUT_MINS: 1
+
+jobs:
+  core_label:
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v4
+      - name: Check if actor is maintainer
+        id: check_maintainer
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const fs = require('fs');
+            const actor = '${{ github.actor }}';
+            const data = await fs.readFileSync('./MAINTAINERS', { encoding: 'utf8' });
+            const maintainers = new Set(data.split('\n'));
+            if (maintainers.has(actor)) {
+              console.log(`🟢 ${actor} is a maintainer`);
+              return true;
+            }
+            console.log(`🔴 ${actor} is NOT a maintainer`);
+            return null;
+      - name: Label PR as React Core Team
+        if: ${{ steps.check_maintainer.outputs.result }}
+        uses: actions/github-script@v7
+        with:
+          script: |
+            github.rest.issues.addLabels({
+              owner: context.repo.owner,
+              repo: context.repo.repo,
+              issue_number: ${{ github.event.number }},
+              labels: ['React Core Team']
+            });

MAINTAINERS

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+acdlite
+bvaughn
+eps1lon
+gaearon
+gnoff
+gsathya
+hoxyq
+jackpope
+jbonta
+josephsavona
+kassens
+lunaleaps
+mattcarrollcode
+mofeiZ
+noahlemen
+poteto
+rickhanlonii
+sebmarkbage
+sethwebster
+sophiebits
+TheSavior
+tyao1
+yuzhi