Make attribute "charset" valid

The attribute "charset" was considered an error, but it results in valid
HTML because HTML is case-insensitive. In other words, these two lines
will be interpreted in the same way:

<meta charSet="utf-8" />
<meta charset="utf-8" />

facebook/react#11492 (comment)

lib/rules/no-unknown-property.js

@@ -117,7 +117,7 @@ const SVGDOM_ATTRIBUTE_NAMES = {
 const DOM_PROPERTY_NAMES = [
   // Standard
   'acceptCharset', 'accessKey', 'allowFullScreen', 'allowTransparency', 'autoComplete', 'autoFocus', 'autoPlay',
-  'cellPadding', 'cellSpacing', 'charSet', 'classID', 'className', 'colSpan', 'contentEditable', 'contextMenu',
+  'cellPadding', 'cellSpacing', 'classID', 'className', 'colSpan', 'contentEditable', 'contextMenu',
   'dateTime', 'encType', 'formAction', 'formEncType', 'formMethod', 'formNoValidate', 'formTarget',
   'frameBorder', 'hrefLang', 'htmlFor', 'httpEquiv', 'inputMode', 'keyParams', 'keyType', 'marginHeight', 'marginWidth',
   'maxLength', 'mediaGroup', 'minLength', 'noValidate', 'onAnimationEnd', 'onAnimationIteration', 'onAnimationStart',

tests/lib/rules/no-unknown-property.js

@@ -30,6 +30,8 @@ ruleTester.run('no-unknown-property', rule, {
     {code: '<App class="bar" />;'},
     {code: '<App for="bar" />;'},
     {code: '<App accept-charset="bar" />;'},
+    {code: '<meta charset="utf-8" />;'},
+    {code: '<meta charSet="utf-8" />;'},
     {code: '<App http-equiv="bar" />;'},
     {code: '<App xlink:href="bar" />;'},
     {code: '<App clip-path="bar" />;'},