[user interface]: Use .png as the XYZ Tile URL format hint for raster & raster-dem tiles

[roblabs]

Launch Checklist

• Briefly describe the changes in this PR.

• When adding a new Raster or Raster-dem tile source, use .png as the hint for the tile URL hint prompt.
  • Currently, the user is prompted for a Vector tile source (e.g., .pbf)
• If a new Mapnik user already has their XYZ raster Tile URL, then they may already know how to modify this text field.
• This change may be useful for new users and is intended for them.
• .png is a start. It could be .webp or .jpg, or .jpg, or in the case of Esri there is no extension for the images.
  • See RobLabs.com/xyz for sample XYZ raster tile URLs

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• Include before/after visuals or gifs if this PR includes visual changes.

• Steps to reproduce
• See RobLabs.com/maputnik for a working version of this fix for Maputnik
  • On top menu bar: Data Sources > In the Add New Source section > Change Source Type to "Raster (Tile URLs)"
    • There you will see prompt hint for a raster Tile instead of vector tile .pbf.
• See the release notes for the National Park Service prototype release for Maputnik for National Park Service Park Tiles for more screen shots.

Before

.pbf may be confusing for those adding a Raster

After

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• Write tests for all new functionality.

• See screen shots for functionality

• [] Add an entry to CHANGELOG.md under the ## main section.
  Not really a feature, so I will leave this out
• Link to related issues.

[HarelM]

Thanks!