.github/workflows/codecov.yml: Initial Codecov.io workflow.

Building in the project root directory so Codecov.io finds the
coverage files. Not sure if it'd work with a separate build/
directory...

Note that the `lcov` package is needed for rom-properties' built-in
coverage target. It technically isn't needed for Codecov.io, but
the build scripts will fail if it's missing.

Root codecov.yml: Ignore extlib/.

NOTE: libromdata is currently built as a shared library in coverage
builds, so LD_LIBRARY_PATH is set in .github/workflows/codecov.yml .

.github/workflows/codecov.yml

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+name: Codecov
+on: [push, pull_request]
+jobs:
+  run:
+    runs-on: ubuntu-latest
+    steps:
+      - name: Checkout
+        uses: actions/checkout@v2
+      - name: Install required packages
+        run: sudo apt-get install --yes cmake ninja-build libcurl4-openssl-dev zlib1g-dev libpng-dev libjpeg-dev nettle-dev pkg-config libtinyxml2-dev gettext libseccomp-dev libzstd-dev liblz4-dev liblzo2-dev qtbase5-dev qttools5-dev-tools extra-cmake-modules libkf5kio-dev libkf5widgetsaddons-dev libkf5filemetadata-dev libglib2.0-dev libgtk-3-dev libcairo2-dev libthunarx-3-dev libgsound-dev libnautilus-extension-dev libgtk-4-dev lcov
+      - name: Run CMake
+        run: cmake . -DCMAKE_BUILD_TYPE=Release -DCMAKE_INSTALL_PREFIX=/usr -DBUILD_TESTING=ON -DENABLE_COVERAGE=ON -G Ninja
+      - name: Run Ninja
+        run: ninja
+      - name: Run tests
+        run: ctest -V -C Release
+        env:
+          LD_LIBRARY_PATH: ${{ github.workspace }}/lib
+      - name: Upload coverage to Codecov
+        uses: codecov/codecov-action@v3
+        env:
+          CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}
+        with:
+          gcov: true

codecov.yml

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+ignore:
+  - "extlib/"