Add GitHub workflow to check Fluent reference files (#5647)

* Add workflow with Fluent linter

* Fix linter error in CircleCI

* Fix path to workflow in paths

* Use explicit requirements.txt file

See actions/setup-python#807
The alternative (creating an empty file) seems worse than using an actual requirements file.

* Reorganize files, address review comments

* Fix pip cache

Author: flodolo

.github/fluent/linter_config.yml

@@ -0,0 +1,24 @@
+# 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
+
+---
+# Check brand names
+CO01:
+  enabled: true
+  brands:
+    - Firefox
+    - Mozilla
+    - Profiler
+  exclusions:
+    files: []
+    messages: []
+# Enforce variable comments
+VC:
+  disabled: false
+# Enforce placeholder style, e.g. { $variable }
+PS01:
+  disabled: false

.github/fluent/requirements.txt

@@ -0,0 +1 @@
+moz-fluent-linter~=0.4.9

.github/workflows/fluent_linter.yml

@@ -0,0 +1,35 @@
+name: Lint Reference Files
+on:
+  push:
+    paths:
+      - 'locales/en-US/*.ftl'
+      - '.github/workflows/fluent_linter.yml'
+      - '.github/fluent/*'
+    branches:
+      - main
+  pull_request:
+    paths:
+      - 'locales/en-US/*.ftl'
+      - '.github/workflows/fluent_linter.yml'
+      - '.github/fluent/*'
+    branches:
+      - main
+  workflow_dispatch:
+jobs:
+  linter:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Clone repository
+        uses: actions/checkout@v5
+      - name: Set up Python 3
+        uses: actions/setup-python@v6
+        with:
+          python-version: '3.11'
+          cache: 'pip'
+          cache-dependency-path: '.github/fluent/requirements.txt'
+      - name: Install Python dependencies
+        run: |
+          pip install -r .github/fluent/requirements.txt
+      - name: Lint reference
+        run: |
+          moz-fluent-lint ./locales/en-US --config .github/fluent/linter_config.yml