Variables: Import Data Sets reads CSV files that are not UTF-8 as Windows-1252

Squashed from #5116 onto main.

crates/engine/src/cmd/variablecmds.rs

@@ -799,7 +799,7 @@ pub fn specs() -> Vec<CommandSpec> {
             "Import Data Sets…",
             ["Window", "Variables"],
             None,
-            "{path? | data? | dataBase64? | csv?} load datasets from CSV, comma- or tab-separated (header: name, then variable names; one row per dataset; without the name column every header is a variable and the rows take the first free names Data Set 1, 2…). A header that names no variable defines one, unbound: `#Name` defines a visibility variable, `@Name` defines an image variable, and a header without a prefix defines a text variable. A prefixed header whose name is a variable of another kind, and a `%` header that isn't a variable's exact name (graph data files), are refused, and so are two headers that name one variable and a header that would define a variable while the variables are locked. Rows whose name already exists are replaced → {imported, updated, names, defined}",
+            "{path? | data? | dataBase64? | csv?} load datasets from CSV, comma- or tab-separated, in UTF-16 with a byte-order mark, UTF-8 or else Windows-1252 (header: name, then variable names; one row per dataset; without the name column every header is a variable and the rows take the first free names Data Set 1, 2…). A header that names no variable defines one, unbound: `#Name` defines a visibility variable, `@Name` defines an image variable, and a header without a prefix defines a text variable. A prefixed header whose name is a variable of another kind, and a `%` header that isn't a variable's exact name (graph data files), are refused, and so are two headers that name one variable and a header that would define a variable while the variables are locked. Rows whose name already exists are replaced → {imported, updated, names, defined}",
             has_doc,
             dataset_import
         ),