Add Geostyler demo page

[geographika]

Description

Adds an HTML page that shows a MapServer WMS on the left, and its equivalent vector layer on the right, styled using SLD from GetStyles requests, GeoStyler, and OpenLayers. This makes it easy to compare MapServer's own rendering with client-side styling of the same layer.

How it works

For the layer selected in the drop-down:

• Left map: the layer is requested as a WMS GetMap image.
• Right map: the features are loaded with a WFS GetFeature request (GeoJSON), and styled in OpenLayers. The style comes from a WMS GetStyles request, which returns the layer's SLD; geostyler-sld-parser reads it and geostyler-openlayers-parser converts it to an OpenLayers style.
• Bottom left: the Mapfile CLASS (and optional SYMBOL) definitions for the layer, loaded from includes/<layer>.class and includes/<layer>.sym.
• Bottom right: the SLD returned by MapServer, formatted in a read-only viewer with folding and search.

Created during the GeoStyler Code Sprint 2026.