Editor: word count: exclude HTML comments and entities

Also make sure `type` is one of the three allowed types and remove unnecessary regex flags.

See #30966.


git-svn-id: https://develop.svn.wordpress.org/trunk@33344 602fd350-edb4-49c9-b593-d223f7449a82

tests/qunit/wp-admin/js/word-count.js

@@ -58,6 +58,20 @@
 				words: 1,
 				characters: 1,
 				all: 1
+			},
+			{
+				message: 'HTML comment.',
+				string: 'one<!-- comment -->two three',
+				words: 2,
+				characters: 11,
+				all: 12
+			},
+			{
+				message: 'HTML entity.',
+				string: '&gt; test',
+				words: 1,
+				characters: 5,
+				all: 6
 			}
 		], function( test ) {
 			_.each( [ 'words', 'characters', 'all' ], function( type ) {