Add configurable text-field property for symbol layers

[birkskyum]

Add configurable text-field property for symbol layers, allowing style definitions to specify which feature property to use for label text (e.g. "name", "name:en", "ref"), instead of the previously hardcoded "name".

💻 Examples

In the style definition, symbol layers can now specify text-field:

{
  "type": "symbol",
  "paint": {
    "text-field": "name"
  }
}

The property flows through: StyleLayer → process_vector → TextTessellatorNew::new(text_field) → PropertyProcessor::property() which matches the configured field name when reading tile features.

🚨 Test instructions

Run cargo run -p maplibre-demo -- headed . Labels should render using the feature property specified by text-field in the style. Changing the value (e.g. to "name:en" or a non-existent field) should change which labels appear.

E.g. in mod.ts, if you change:

// mod.ts 148:159
                StyleLayer {
                    index: 9,
                    id: "text".to_string(),
                    maxzoom: None,
                    minzoom: None,
                    metadata: None,
                    paint: Some(LayerPaint::Symbol(SymbolPaint {
                        text_field: Some("name:en".to_string()), // <- name:en or name:da will control label language
                    })),
                    source: None,
                    source_layer: Some("place".to_string()),
                },

name:da - København

name:en - Copenhagen