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

Squashed from #5116 onto main.

docs/mcp.md

@@ -1658,6 +1658,9 @@ already walks Image Trace. Short agent entry points:
   data files) is refused, and so are two cells that name one variable (`City` twice, or `Shown` and `#Shown`). The
   reply's `defined` lists the variables the import defined. While the variables are
   locked, an import whose header would define a variable fails with "variables are locked".
+  A `dataset.import` file that isn't UTF-8 is read as Windows-1252, unless it starts with the UTF-8 byte-order mark.
+  A file that starts with a UTF-16 byte-order mark is read as UTF-16, and a file without one that holds NUL bytes is
+  refused. `csv` and `data` are read as the text they hold.
 - Variable libraries: `variable.loadLibrary {path|xml|dataBase64}` (the Variables panel menu's Load Variable
   Library…) replaces the document's variables and data sets with a variable library's in one undo step. A load, like a
   `dataset.import`, adds no undo step when it leaves the variables and data sets as they were. The library