Add custom syntax for tagging spoilers

Currently the only two ways to spoiler text in your message is either:
* Using the /spoiler command
* Manually typing the data-mx-spoiler span HTML blocks

Neither one is discoverable, or friendly to users really. Instead, we
should extend our existing Markdown-based formatting syntax with one
that can handle spoiler tags. I chose the || syntax to match Discord,
since Element doesn't seem to adopt one.

Unfortunately, CMark does not support custom extensions (see
https://github.com/commonmark/cmark/pull/123) so we have to implement
our own parsing function. New tests are also added for this too.

src/libneochat/texthandler.cpp

@@ -53,6 +53,7 @@ QString TextHandler::handleSendText()
 {
     m_pos = 0;
     m_dataBuffer = markdownToHTML(m_data);
+    m_dataBuffer = customMarkdownToHtml(m_dataBuffer);
 
     m_nextTokenType = nextTokenType(m_dataBuffer, m_pos, m_nextToken, m_nextTokenType);
 
@@ -701,6 +702,31 @@ QString TextHandler::linkifyUrls(QString stringIn)
     return stringIn;
 }
 
+QString TextHandler::customMarkdownToHtml(const QString &stringIn)
+{
+    QString buffer = stringIn;
+
+    while (true) {
+        const int pos = buffer.indexOf(u"||"_s);
+        if (pos == -1) {
+            break;
+        }
+
+        int nextPos = buffer.indexOf(u"||"_s, pos + 1);
+        if (nextPos == -1) {
+            break;
+        }
+
+        buffer.replace(pos, 2, QStringLiteral("<span data-mx-spoiler>"));
+
+        // we have to re-search because the index moved!
+        nextPos = buffer.indexOf(u"||"_s, pos + 1);
+        buffer.replace(nextPos, 2, QStringLiteral("</span>"));
+    }
+
+    return buffer;
+}
+
 QString TextHandler::editString() const
 {
     Kirigami::Platform::PlatformTheme *theme =

src/libneochat/texthandler.h

@@ -140,6 +140,7 @@ private:
     QString escapeHtml(QString stringIn);
     QString unescapeHtml(QString stringIn);
     QString linkifyUrls(QString stringIn);
+    QString customMarkdownToHtml(const QString &stringIn);
 
     QString editString() const;
     QString emoteString(const NeoChatRoom *room = nullptr, const Quotient::RoomEvent *event = nullptr) const;