slopes 2.0.0

• Migrated from the deprecated raster package to terra (closes #60).
  terra is now a hard dependency (Imports); raster is no longer required or suggested.
• elevation_extract(), slope_raster(), and elevation_add() now always use
  terra. The terra = argument is deprecated (with a message) and ignored.
• elevation_get() now always returns a SpatRaster. The output_format =
  argument is deprecated and ignored.
• slope_matrix_to_raster() and slope_xyz_simple() now work with SpatRaster
  objects. Legacy Raster* objects are accepted with an automatic conversion
  message for backwards compatibility.
• The bundled dataset has been renamed from dem_lisbon_raster to dem_lisbon
  and is now a SpatRaster object (was a RasterLayer).
• Fixed CRAN --run-donttest failure: the ceramic/Mapbox-dependent example in
  elevation_add() is now wrapped in \dontrun{} instead of \donttest{} (closes #72).
• Fixed slopes_palette() hardcoded testing logic, allowing it to correctly return any number of requested colors for the "Green-Brown" palette.
• Substantially improved overall package test coverage (from ~59% to over 84%) by adding test wrappers for plot functions and matrix conversion utilities.