Additional flags for commands from csv and from tsv (#8398)

# Description

Resolves issue #8370

Adds the following flags to commands `from csv` and `from tsv`:
- `--flexible`: allow the number of fields in records to be variable
- `-c --comment`: a comment character to ignore lines starting with it
- `-q --quote`: a quote character to ignore separators in strings,
defaults to '\"'
- `-e --escape`: an escape character for strings containing the quote
character

Internally, the `Value` struct has an additional helper function
`as_char` which converts it to a single `char`

# User-Facing Changes

The single quoted string `'\t'` can no longer be used as a parameter for
the flag `--separator '\t'` as it is interpreted as a two-character
string. One needs to use from now on the flag with a double quoted
string like so: `-s "\t"` which correctly interprets the string as a
single `char`.

crates/nu-protocol/src/value/mod.rs

@@ -190,6 +190,27 @@ impl Clone for Value {
 }
 
 impl Value {
+    pub fn as_char(&self) -> Result<char, ShellError> {
+        match self {
+            Value::String { val, span } => {
+                let mut chars = val.chars();
+                match (chars.next(), chars.next()) {
+                    (Some(c), None) => Ok(c),
+                    _ => Err(ShellError::MissingParameter {
+                        param_name: "single character separator".into(),
+                        span: *span,
+                    }),
+                }
+            }
+            x => Err(ShellError::CantConvert {
+                to_type: "char".into(),
+                from_type: x.get_type().to_string(),
+                span: self.span()?,
+                help: None,
+            }),
+        }
+    }
+
     /// Converts into string values that can be changed into string natively
     pub fn as_string(&self) -> Result<String, ShellError> {
         match self {