Add a lint check to complain about trailing whitespace

As mentioned below, this whitespace can be annoying because some editors like to delete it by
default, causing spurious diffs if someone is working on an unrelated thing. Let's avoid this by
standardizing on no trailing whitespace

run-lint

@@ -1,5 +1,6 @@
 #!/bin/sh
 
+space=' '
 tab='	'
 ok=yes
 
@@ -22,11 +23,19 @@ for file in \
     examples/*/*.lua examples/*/*.pln \
     vm/src/pallene_core.c vm/src/pallene_core.h
 do
-    if grep "^$tab" "$file"; then
+    if grep --line-number "[$space$tab]$" "$file"; then
+        # Forbid trailing whitespace because some editors like to automatically delete it.
+        # Such whitespace can cause spurious diffs later down the road, when someone is working on
+        # an unrelated pull request and their editor "helpfully" deletes the trailing whitespace.
+        error "File $file has a line that ends in whitespace"
+    fi
+
+    if grep --line-number "^$tab" "$file"; then
+        # Standardize on spaces because mixing tabs and spaces is endless pain.
         error "File $file has tab-based indentation"
     fi
 
-    if ! grep --quiet 'SPDX-License-Identifier' "$file"; then
+    if ! grep --line-number --quiet 'SPDX-License-Identifier' "$file"; then
         error "File $file is missing a copyright header"
     fi
 done