Add fenced code block style rule (#224) (fixes #223).

lib/md048.js

@@ -0,0 +1,38 @@
+// @ts-check
+
+"use strict";
+
+const { addErrorDetailIf } = require("../helpers");
+
+function fencedCodeBlockStyleFor(markup) {
+  switch (markup[0]) {
+    case "~":
+      return "tilde";
+    default:
+      return "backtick";
+  }
+}
+
+module.exports = {
+  "names": [ "MD048", "code-fence-style" ],
+  "description": "Code fence style",
+  "tags": [ "code" ],
+  "function": function MD048(params, onError) {
+    const style = params.config.style || "consistent";
+    let expectedStyle = style;
+    params.tokens
+      .filter((token) => token.type === "fence")
+      .forEach((fenceToken) => {
+        const { lineNumber, markup } = fenceToken;
+        if (expectedStyle === "consistent") {
+          expectedStyle = fencedCodeBlockStyleFor(markup);
+        }
+        addErrorDetailIf(
+          onError,
+          lineNumber,
+          expectedStyle,
+          fencedCodeBlockStyleFor(markup)
+        );
+      });
+  }
+};