createSpatialIndexSql function
Builds the SQL for CreateSpatialIndex, which creates the R*Tree spatial
index (idx_<table>_<column>) SpatiaLite uses to accelerate bounding-box
filtering for the given geometry column.
Must be called after addGeometryColumnSql has been executed for the
same table/column.
Implementation
String createSpatialIndexSql(String table, String column) =>
"SELECT CreateSpatialIndex('$table', '$column')";