feat: allow from csv to accept 4 byte unicode separator chars (#10138)

- this PR should close #10132

# Description
* added a flag to `from csv --ascii` that replaces the given `separator
with the unicode separator x1f https://www.codetable.net/hex/1f (aka
Information Separator One)

# User-Facing Changes
New flags are available for `from csv` ( `--ascii` or short `-a`)

# Tests + Formatting
There are no tests at the moment. Code has been formatted.
- `cargo test --workspace` (breaks with a non related test on my
machine)

crates/nu-command/src/formats/from/csv.rs

@@ -21,7 +21,7 @@ impl Command for FromCsv {
             .named(
                 "separator",
                 SyntaxShape::String,
-                "a character to separate columns, defaults to ','",
+                "a character to separate columns (either single char or 4 byte unicode sequence), defaults to ','",
                 Some('s'),
             )
             .named(
@@ -134,11 +134,22 @@ fn from_csv(
 ) -> Result<PipelineData, ShellError> {
     let name = call.head;
 
-    let separator = call
-        .get_flag(engine_state, stack, "separator")?
-        .map(|v: Value| v.as_char())
-        .transpose()?
-        .unwrap_or(',');
+    let separator = match call.get_flag::<String>(engine_state, stack, "separator")? {
+        Some(sep) => {
+            if sep.len() == 1 {
+                sep.chars().next().unwrap_or(',')
+            } else if sep.len() == 4 {
+                let unicode_sep = u32::from_str_radix(&sep, 16);
+                char::from_u32(unicode_sep.unwrap_or(b'\x1f' as u32)).unwrap_or(',')
+            } else {
+                return Err(ShellError::NonUtf8Custom(
+                    "separator should be a single char or a 4-byte unicode".to_string(),
+                    call.span(),
+                ));
+            }
+        }
+        None => ',',
+    };
     let comment = call
         .get_flag(engine_state, stack, "comment")?
         .map(|v: Value| v.as_char())