[ci] Automatically label PRs from core team

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.

.github/workflows/shared_core_label.yml

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+name: (Shared) Core Label
+
+on:
+  pull_request:
+
+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:
+  notify:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Check if actor is maintainer
+        id: check_maintainer
+        uses: actions/github-script@v7
+        with:
+          script: |
+            const fs = require('fs/promises');
+
+            async function getMaintainers() {
+              const data = await fs.readFile('./MAINTAINERS', { encoding: 'utf8' });
+              return new Set(data.split('\n'));
+            }
+
+            async function main() {
+              const maintainers = await getMaintainers();
+              if (maintainers.has(github.event.actor)) {
+                return true;
+              }
+              return false;
+            }
+      - if: ${{ steps.check_maintainer.outputs.result == true }}
+        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