ci: add codestyle check

.github/workflows/codestyle.yml

@@ -0,0 +1,15 @@
+name: check-codestyle
+on:
+  pull_request:
+
+jobs:
+  check-codestyle:
+    strategy:
+      fail-fast: false
+    runs-on: ubuntu-latest
+    name: check codestyle
+    steps:
+      - uses: actions/checkout@v2
+      - name: Check core codestyle
+        run: source ./codestyle.sh
+

codestyle.sh

@@ -0,0 +1,23 @@
+#!/bin/bash
+set -e
+
+echo "Codestyle check script:"
+echo
+
+declare -A singleLineRegexChecks=(
+    ["[[:blank:]]$"]="Remove whitespace at the end of the lines above"
+    ["\t"]="Replace tabs with 4 spaces in the lines above"
+)
+
+for check in ${!singleLineRegexChecks[@]}; do
+    echo "  Checking RegEx: '${check}'"
+
+    if grep -P -r -I -n ${check} exercises/ | grep .cpp; then
+        echo
+        echo "${singleLineRegexChecks[$check]}"
+        exit 1
+    fi
+done
+
+echo
+echo "Everything looks good"