[new plugin] remove-xlink-ns

Ref #1508

The new plugin removes xmlns:xlink from <svg> and replaces xlink:href
with href attribute.

xlink namespace is obsolete in SVG 2. Href attribute is recommended
replacement to xlink:href.

This plugin will be enabled by default in SVGO 3.

See https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/href

plugins/plugins.js

@@ -4,6 +4,7 @@
 exports['preset-default'] = require('./preset-default.js');
 
 // builtin plugins
+exports['remove-xlink-ns'] = require('./remove-xlink-ns.js');
 exports.addAttributesToSVGElement = require('./addAttributesToSVGElement.js');
 exports.addClassesToSVGElement = require('./addClassesToSVGElement.js');
 exports.cleanupAttrs = require('./cleanupAttrs.js');

plugins/remove-xlink-ns.js

@@ -0,0 +1,33 @@
+'use strict';
+
+exports.name = 'remove-xlink-ns';
+exports.type = 'visitor';
+exports.active = false;
+exports.description =
+  'removes xmlns:xlink and replaces xlink:href with href attribute';
+
+/**
+ * removes xmlns:xlink from <svg> and replaces xlink:href with href attribute
+ *
+ * xlink namespace is obsolete in SVG 2. Href attribute is recommended
+ * replacement to xlink:href.
+ *
+ * https://developer.mozilla.org/en-US/docs/Web/SVG/Attribute/href
+ *
+ * @type {import('../lib/types').Plugin<void>}
+ */
+exports.fn = () => {
+  return {
+    element: {
+      enter: (node) => {
+        if (node.attributes['xmlns:xlink'] != null) {
+          delete node.attributes['xmlns:xlink'];
+        }
+        if (node.attributes['xlink:href'] != null) {
+          node.attributes['href'] = node.attributes['xlink:href'];
+          delete node.attributes['xlink:href'];
+        }
+      },
+    },
+  };
+};