Add workflow to lint Fluent en-US files

.github/fluent_linter_config.yml

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+# This Source Code Form is subject to the terms of the Mozilla Public
+# License, v. 2.0. If a copy of the MPL was not distributed with this
+# file, You can obtain one at http://mozilla.org/MPL/2.0/.
+
+# See https://github.com/mozilla-l10n/moz-fluent-linter/blob/main/src/fluent_linter/config.yml
+# for details
+
+---
+ID01:
+    enabled: true
+    exclusions:
+        messages: []
+        files: []
+ID02:
+    enabled: true
+    min_length: 9
+    exclusions:
+        messages: []
+        files: []
+CO01:
+    enabled: true
+    brands:
+        - Firefox
+        - Mozilla
+    exclusions:
+        files: []
+        messages: []

.github/requirements.txt

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+moz-fluent-linter==0.4.*

.github/workflows/fluent_linter.yml

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+name: Lint Fluent Reference Files
+on:
+  push:
+    paths:
+      - 'l10n/en-US/**.ftl'
+      - '.github/fluent_linter_config.yml'
+      - '.github/workflows/fluent_linter.yaml'
+    branches:
+      - master
+  pull_request:
+    paths:
+      - 'l10n/en-US/**.ftl'
+      - '.github/fluent_linter_config.yml'
+      - '.github/workflows/fluent_linter.yaml'
+    branches:
+      - master
+  workflow_dispatch:
+permissions:
+  contents: read
+
+jobs:
+  linter:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Clone repository
+        uses: actions/checkout@v4
+      - name: Set up Python 3
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.10'
+          cache: 'pip'
+      - name: Install Python dependencies
+        run: |
+          pip install -r .github/requirements.txt
+      - name: Lint reference
+        run: |
+          moz-fluent-lint ./l10n/en-US --config .github/linter_config.yml