Update "Add Raster Layer" to support `{zoom}` as an alternative to `{z}`

Request: Support `{zoom}` as an alternative to `{z}` in the "Add Raster Layer" feature.

Many tile providers are unable to be used as they expect `{zoom}`.

Refer to the [Editor Layer Index](https://osmlab.github.io/editor-layer-index/) where searching for "{zoom}" returns ~800 results while "{z}" returns none.

Example: `https://gismaps.kingcounty.gov/arcgis/rest/services/BaseMaps/KingCo_Aerial_2025/MapServer/WMTS/tile/1.0.0/BaseMaps_KingCo_Aerial_2025/default/default028mm/{zoom}/{y}/{x}`

_Image: "Add Raster Layer" dialog box, with a URL including `{zoom}` entered and the error message "Must include {z}, {x}, and {y} (or {-y} for TMS) placeholders, or use a WMS URL with {bbox-epsg-3857}" visible._

Start at the "Add Raster Layer" feature and locate the validation that requires {z}, {x}, and {y} placeholders. Update the relevant validation and its error wording so {zoom} is accepted as an alternative to {z}, while existing {z} and WMS URLs continue to work.

Written by the indexing model from the issue text.