Fold indicator icons should support VS Code style out-of-the-box

[bobbylight]

Describe the solution you'd like
RSyntaxTextArea's fold indicator icons are standard squares and resemble the fold indicators in Visual Studio. Newer applications, such as VS Code, use a simpler, arrow-style icon, which often times looks more modern.

While the icons RSTA uses in its gutter are customizable, it would be nice if the API provided a way to toggle between these two icon formats directly (and possibly more in the future). The reason this should be baked into the library is that it's easier for us to provide icons that use the RSTA Theme, and respond to changes to it, more easiliy than it is for clients providing raw Icon implementations via the existing API.

Are there any workarounds?
Use Gutter.setFoldIcons(Icon, Icon) with a homebrew icon implementation.

Additional context
None.

[bobbylight]

Closing as a duplicate of #458 .