Add EditorConfig rules, fix trailing whitespaces

.github/workflows/editorconfig.yml

@@ -0,0 +1,20 @@
+name: Check EditorConfig
+
+on:
+  pull_request:
+  push:
+jobs:
+  build:
+    runs-on: ubuntu-latest
+
+    steps:
+      - uses: actions/checkout@v2.3.4
+      - uses: cachix/install-nix-action@v15
+
+      - name: Enable binary cache (optional)
+        uses: cachix/cachix-action@v10
+        with:
+          name: colmena
+          authToken: '${{ secrets.CACHIX_AUTH_TOKEN }}'
+
+      - run: nix-shell --run "editorconfig-checker"