Introduce a GitHub Actions workflow for running the font tests

This commit migrates the font tests away from the bots. Not only are the
font tests broken on the Windows bot since some time, they also run on
Python 2 (end of life since January 2020) and `ttx` 3.19.0 (released in
November 2017). The latter is installed via a submodule, which requires
more complicated logic for finding and running `ttx`.

We solve the issues by implementing a modern workflow that installs the
most recent stable Python and `ttx` (`fonttools` package) versions. This
simplifies the `ttx` driver code as well because it can now assume `ttx`
is available on the path (just like we do for e.g. `node` invocations).
GitHub Actions takes care of creating a virtual environment with
`fonttools` in it so that the `ttx`  entrypoint is available. Locally
the font tests can be run in a similar way by creating and sourcing a
virtual environment with `fonttools` in it before running the font
tests, and a README file is included with instructions for doing so.

.github/workflows/font_tests.yml

@@ -0,0 +1,64 @@
+name: Font tests
+on:
+  push:
+    paths:
+      - 'gulpfile.mjs'
+      - 'src/**'
+      - 'test/test.mjs'
+      - 'test/font/**'
+      - '.github/workflows/font_tests.yml'
+    branches:
+      - master
+  pull_request:
+    paths:
+      - 'gulpfile.mjs'
+      - 'src/**'
+      - 'test/test.mjs'
+      - 'test/font/**'
+      - '.github/workflows/font_tests.yml'
+    branches:
+      - master
+  workflow_dispatch:
+permissions:
+  contents: read
+
+jobs:
+  test:
+    name: Test
+
+    strategy:
+      fail-fast: false
+      matrix:
+        node-version: [lts/*]
+        os: [windows-latest, ubuntu-latest]
+
+    runs-on: ${{ matrix.os }}
+
+    steps:
+      - name: Checkout repository
+        uses: actions/checkout@v4
+        with:
+          fetch-depth: 0
+
+      - name: Use Node.js ${{ matrix.node-version }}
+        uses: actions/setup-node@v4
+        with:
+          node-version: ${{ matrix.node-version }}
+
+      - name: Install Gulp
+        run: npm install -g gulp-cli
+
+      - name: Install other dependencies
+        run: npm install
+
+      - name: Use Python 3.12
+        uses: actions/setup-python@v4
+        with:
+          python-version: '3.12'
+          cache: 'pip'
+
+      - name: Install Fonttools
+        run: pip install fonttools
+
+      - name: Run font tests
+        run: gulp fonttest --headless