feat(CI): check for trailing whitespace

Run on pull request only.
Use a shell script to check for trailing whitespace in all files.
Fail if trailing whitespace is found.

.github/workflows/lint.yml

@@ -31,3 +31,12 @@ jobs:
           node-version: '12.x'
       - run: npm install -g markdownlint-cli@0.25.0
       - run: markdownlint '**/*.md'
+  trailing-whitespace:
+    name: 🌌 Trailing whitespace
+    runs-on: ubuntu-latest
+    steps:
+      - uses: actions/checkout@v2
+      - name: 🧹 Check for trailing whitespace
+        run: |
+          cd test || exit
+          sh ./check-for-trailing-whitespace.sh || exit 1

mrbgems/mruby-hash-ext/src/hash-ext.c

@@ -74,7 +74,7 @@ hash_slice(mrb_state *mrb, mrb_value hash)
  *     hsh.except(*keys) -> a_hash
  *
  *  Returns a hash excluding the given keys and their values.
- * 
+ *
  *     h = { a: 100, b: 200, c: 300 }
  *     h.except(:a)          #=> {:b=>200, :c=>300}
  *     h.except(:b, :c, :d)  #=> {:a=>100}

test/check-for-trailing-whitespace.sh

+#!/usr/bin/env bash
+
+cd .. || exit
+# print first
+grep -EHInr '( +)$' ./*
+
+var=$(grep -EHInr '( +)$' ./*)
+# then exit with fail if found
+if test -z "$var"; then
+  exit 0
+else
+  exit 1
+fi