add code highlight to RichTextEditor

[AnnMarieW]

This PR adds code highlighting support to RichTextEditor, consistent with the Mantine example

Notes

• by default, the extension "CodeBlockLowlight" is enabled.

• To keep the bundle size low only a few languages are included ( ts, js, python, css, text)

• When users customize the extensions prop, they must explicitly include "CodeBlockLowlight" to keep code highlighting support.

examples:

     # by default all extensions are included
    dmc.RichTextEditor()

    # This has customized extensions and only StarterKit is enabled.  No code highlighting
   dmc.RichTextEditor(
       extensions=["StarterKit"]
    )

    #  This customized extension has code highlighting 
     dmc.RichTextEditor(
       extensions=["StarterKit", "CodeBlockLowlight"]
    )

Example App

from dash import Dash
import dash_mantine_components as dmc
app = Dash()

component= dmc.RichTextEditor(
    html="",
    extensions=["StarterKit", "CodeBlockLowlight"],
    toolbar={
        "controlsGroups": [
            [
                "Bold",
                "Italic",
                "Underline",
                "Strikethrough",
                "CodeBlock"
            ],
        ],
    },   
)

app.layout = dmc.MantineProvider(
    component
)

if __name__ == "__main__":
    app.run(debug=True)