CI: add a codespell job

Author: bilelmoussaoui

.github/workflows/codespell.yml

@@ -0,0 +1,18 @@
+on:
+  push:
+    branches: [master]
+  pull_request:
+
+name: Codespell
+
+jobs:
+  codespell:
+    runs-on: ubuntu-20.04
+    steps:
+      - uses: actions/checkout@v2
+      - uses: codespell-project/actions-codespell@master
+        with:
+          check_filenames: true
+          skip: "./.git/**,./flat-manager/dist/index.js,./flatpak-builder/dist/index.js,**/yarn.lock"
+          builtin: "clear,rare,informal,usage,code"
+          ignore_words_list: "master" # can't do much about it, flatpak uses it by default