Add code syntax highlight utils (#88)

* add code syntax highlight code

* add example

* add documentation and fix example

* add tests

* add information about napari theme usage

* clean napari mention

examples/code_highlight.py

@@ -0,0 +1,32 @@
+from PyQt5.QtGui import QColor, QPalette
+from qtpy.QtWidgets import QApplication, QTextEdit
+
+from superqt.utils import CodeSyntaxHighlight
+
+app = QApplication([])
+
+text_area = QTextEdit()
+
+highlight = CodeSyntaxHighlight(text_area.document(), "python", "monokai")
+
+palette = text_area.palette()
+palette.setColor(QPalette.Base, QColor(highlight.background_color))
+text_area.setPalette(palette)
+text_area.setText(
+    """from argparse import ArgumentParser
+
+def main():
+    parser = ArgumentParser()
+    parser.add_argument("name", help="Your name")
+    args = parser.parse_args()
+    print(f"Hello {args.name}")
+
+
+if __name__ == "__main__":
+    main()
+"""
+)
+
+text_area.show()
+
+app.exec_()