Utils: add Position::fromCursor with tests

Change-Id: I1cd989eaf7e75bc04f171989f9f9fe932402abef
Reviewed-by: Qt CI Bot <qt_ci_bot@qt-project.org>
Reviewed-by: Jarek Kobus <jaroslaw.kobus@qt.io>

src/libs/utils/textutils.cpp

@@ -59,6 +59,11 @@ Position Position::fromPositionInDocument(const QTextDocument *document, int pos
     return {};
 }
 
+Position Position::fromCursor(const QTextCursor &c)
+{
+    return c.isNull() ? Position{} : Position{c.blockNumber() + 1, c.positionInBlock()};
+}
+
 int Range::length(const QString &text) const
 {
     if (end.line < begin.line)