Set Snowflake escape char to backslash

[turb]

Set Snowflake escape char to backslash since it is the default used by CSVParser (fixes #24467).

One improvement would be to have this as configuration, but the change would be quite important.

It does not fix a related problem: it fails in some cases with \n in the data.

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[damondouglas]

Thank you @turb for your help!