LibHTML+IRCClient: Add an escape_html_entities() helper

This simple helper escapes '<', '>' and '&' so they can be used in HTML
text without interfering with the parser.

Use this in IRCClient to prevent incoming messages from messing with
the DOM :^)

Applications/IRCClient/IRCLogBuffer.cpp

@@ -54,7 +54,7 @@ void IRCLogBuffer::add_message(char prefix, const String& name, const String& te
         color.to_string().characters(),
         timestamp_string().characters(),
         nick_string.characters(),
-        text.characters());
+        escape_html_entities(text).characters());
     auto fragment = parse_html_fragment(*m_document, html);
     m_container_element->append_child(fragment->remove_child(*fragment->first_child()));
     m_document->force_layout();
@@ -69,7 +69,7 @@ void IRCLogBuffer::add_message(const String& text, Color color)
         "</div>",
         color.to_string().characters(),
         timestamp_string().characters(),
-        text.characters());
+        escape_html_entities(text).characters());
     auto fragment = parse_html_fragment(*m_document, html);
     m_container_element->append_child(fragment->remove_child(*fragment->first_child()));
     m_document->force_layout();

Libraries/LibHTML/Parser/HTMLParser.cpp

@@ -339,3 +339,19 @@ RefPtr<Document> parse_html_document(const StringView& html, const URL& url)
 
     return document;
 }
+
+String escape_html_entities(const StringView& html)
+{
+    StringBuilder builder;
+    for (int i = 0; i < html.length(); ++i) {
+        if (html[i] == '<')
+            builder.append("&lt;");
+        else if (html[i] == '>')
+            builder.append("&gt;");
+        else if (html[i] == '&')
+            builder.append("&amp;");
+        else
+            builder.append(html[i]);
+    }
+    return builder.to_string();
+}

Libraries/LibHTML/Parser/HTMLParser.h

@@ -7,3 +7,4 @@ class DocumentFragment;
 
 RefPtr<Document> parse_html_document(const StringView&, const URL& = URL());
 RefPtr<DocumentFragment> parse_html_fragment(Document&, const StringView&);
+String escape_html_entities(const StringView&);