Meta: Enforce Prettier formatting of Markdown files

We already use the opinionated Prettier formatter
for JavaScript (with appropriate configuration),
and in general it seems to be one of the best
Markdown auto-formatters. Therefore, the choice
seems natural to finally enforce Markdown
formatting.

Meta/lint-prettier.sh

@@ -10,20 +10,20 @@ if [ "$#" -eq "0" ]; then
         git ls-files \
             --exclude-from .prettierignore \
             -- \
-            '*.js' '*.mjs'
+            '*.js' '*.mjs' '*.md'
     )
 else
     files=()
     for file in "$@"; do
-        if [[ "${file}" == *".js" ]] || [[ "${file}" == *".mjs" ]]; then
+        if [[ "${file}" == *".js" || "${file}" == *".mjs" || "${file}" == *".md" ]]; then
             files+=("${file}")
         fi
     done
 fi
 
 if (( ${#files[@]} )); then
     if ! command -v prettier >/dev/null 2>&1 ; then
-        echo "prettier is not available, but JS files need linting! Either skip this script, or install prettier."
+        echo "prettier is not available, but JS or Markdown files need linting! Either skip this script, or install prettier."
         exit 1
     fi
 
@@ -34,5 +34,5 @@ if (( ${#files[@]} )); then
 
     prettier --check "${files[@]}"
 else
-    echo "No .js or .mjs files to check."
+    echo "No .js, .mjs, or .md files to check."
 fi