ESM support

[aderchox]

Hi, I've added ESM support to this package. Things done in this commit:
- Converts CommonJS module.exports to ESM export using a Rollup plugin
- Adds an ESM import to the example in README.md
- Adds the change in HISTORY.md
- Adds "exports" key in package.json to detect proper export automatically
I don't know Travis CI though, that should probably be updated too so that index.mjs will not be ignored when installing the package using NPM. Thank you.

[WofWca]

I have confirmed that the new file only contains gluing and formatting changes (if you add semicolons, the differences go away). This means that the functionality is the same.

diff

@@ -6,19 +6,21 @@
  * MIT Licensed
  */
 
+'use strict'
+
 /**
  * Module variables.
  * @private
  */
 
-var matchHtmlRegExp = /["'&<>]/;
+var matchHtmlRegExp = /["'&<>]/
 
 /**
  * Module exports.
  * @public
  */
 
-var oss = escapeHtml;
+module.exports = escapeHtml
 
 /**
  * Escape special characters in the given string of text.
@@ -29,50 +31,48 @@ var oss = escapeHtml;
  */
 
 function escapeHtml (string) {
-  var str = '' + string;
-  var match = matchHtmlRegExp.exec(str);
+  var str = '' + string
+  var match = matchHtmlRegExp.exec(str)
 
   if (!match) {
     return str
   }
 
-  var escape;
-  var html = '';
-  var index = 0;
-  var lastIndex = 0;
+  var escape
+  var html = ''
+  var index = 0
+  var lastIndex = 0
 
   for (index = match.index; index < str.length; index++) {
     switch (str.charCodeAt(index)) {
       case 34: // "
-        escape = '&quot;';
+        escape = '&quot;'
         break
       case 38: // &
-        escape = '&amp;';
+        escape = '&amp;'
         break
       case 39: // '
-        escape = '&#39;';
+        escape = '&#39;'
         break
       case 60: // <
-        escape = '&lt;';
+        escape = '&lt;'
         break
       case 62: // >
-        escape = '&gt;';
+        escape = '&gt;'
         break
       default:
         continue
     }
 
     if (lastIndex !== index) {
-      html += str.substring(lastIndex, index);
+      html += str.substring(lastIndex, index)
     }
 
-    lastIndex = index + 1;
-    html += escape;
+    lastIndex = index + 1
+    html += escape
   }
 
   return lastIndex !== index
     ? html + str.substring(lastIndex, index)
     : html
-}
-
-export { oss as default };
\ No newline at end of file
+}
\ No newline at end of file