Update MD051/link-fragments to add ignore_case parameter and improve documentation (fixes #1274).

lib/configuration-strict.d.ts

@@ -937,11 +937,25 @@ export interface ConfigurationStrict {
   /**
    * MD051/link-fragments : Link fragments should be valid : https://github.com/DavidAnson/markdownlint/blob/v0.35.0/doc/md051.md
    */
-  MD051?: boolean;
+  MD051?:
+    | boolean
+    | {
+        /**
+         * Ignore case of fragments
+         */
+        ignore_case?: boolean;
+      };
   /**
    * MD051/link-fragments : Link fragments should be valid : https://github.com/DavidAnson/markdownlint/blob/v0.35.0/doc/md051.md
    */
-  "link-fragments"?: boolean;
+  "link-fragments"?:
+    | boolean
+    | {
+        /**
+         * Ignore case of fragments
+         */
+        ignore_case?: boolean;
+      };
   /**
    * MD052/reference-links-images : Reference links and images should use a label that is defined : https://github.com/DavidAnson/markdownlint/blob/v0.35.0/doc/md052.md
    */

lib/md051.js

@@ -2,7 +2,7 @@
 
 "use strict";
 
-const { addError, addErrorDetailIf, getHtmlAttributeRe } = require("../helpers");
+const { addError, getHtmlAttributeRe } = require("../helpers");
 const { filterByPredicate, filterByTypes, getHtmlTagInfo } = require("../helpers/micromark-helpers.cjs");
 const { filterByTypesCached } = require("./cache");
 
@@ -68,6 +68,7 @@ module.exports = {
   "tags": [ "links" ],
   "parser": "micromark",
   "function": function MD051(params, onError) {
+    const ignoreCase = params.config.ignore_case || false;
     const fragments = new Map();
 
     // Process headings
@@ -143,16 +144,16 @@ module.exports = {
             if (mixedCaseKey) {
               // @ts-ignore
               (fixInfo || {}).insertText = mixedCaseKey;
-              addErrorDetailIf(
-                onError,
-                link.startLine,
-                mixedCaseKey,
-                text,
-                undefined,
-                context,
-                range,
-                fixInfo
-              );
+              if (!ignoreCase && (mixedCaseKey !== text)) {
+                addError(
+                  onError,
+                  link.startLine,
+                  `Expected: ${mixedCaseKey}; Actual: ${text}`,
+                  context,
+                  range,
+                  fixInfo
+                );
+              }
             } else {
               addError(
                 onError,