Split lib and demo

[forman]

JS

• Reorganised Chartlets project to better separate demo from library code.
  Using monorepo layout for chartlets.js with workspaces lib and demo
  that host the packages for chartlets and chartlets-demo.

• Chartlets now allows for plugins that can provide individual component
  implementations.
  The Vega-based chart and MUI components are now optional and have been
  moved into respective plugin modules chartlets/plugins/vega and
  chartlets/plugins/mui.
  To activate them, use the new plugins: PluginLike[] option
  of FrameworkOptions:

  import { configureFramework } from "chartlets";
  import mui from "chartlets/plugins/mui";
  import vega from "chartlets/plugins/vega";
  
  configureFramework({ plugins: [mui(), vega()], ... });   

• Renamed Plot component into VegaChart.

• The new VegaChart component respects a theme property. If not given,
  it will respect the current theme mode "dark" otherwise fallback to the
  Vega default theme.

• The demo application now allows for switching the theme mode between
  dark, light, and system mode.

Python

• Reorganised Chartlets project to better separate demo from library code.
  Created separate folder demo in chartlets.py that contains
  a demo server package and example configuration.
  Also simplified demo server code:

  • Moved panel module one level up
  • Removed util module which was no longer required

• Allow for different chart providers. VegaChart can be configured only if
  altair package is installed.

• Renamed Plot into VegaChart, which now also respects a theme property.