support plus sign for "into filesize" (#12974)

# Description
Fixes https://github.com/nushell/nushell/issues/12968. After apply this
patch, we can use explict plus sign character included string with `into
filesize` cmd.

# User-Facing Changes
AS-IS (before fixing)
```
$ "+8 KiB" | into filesize                                                                                                         
Error: nu:🐚:cant_convert

  × Can't convert to int.
   ╭─[entry #31:1:1]
 1 │ "+8 KiB" | into filesize
   · ────┬───
   ·     ╰── can't convert string to int
   ╰────
```

TO-BE (after fixing)
```
$ "+8KiB" | into filesize                                                                                       
8.0 KiB
```

# Tests + Formatting
Added a test 

# After Submitting
<!-- If your PR had any user-facing changes, update [the
documentation](https://github.com/nushell/nushell.github.io) after the
PR is merged, if necessary. This will help us keep the docs up to date.
-->

crates/nu-command/src/conversions/into/filesize.rs

@@ -138,6 +138,7 @@ pub fn action(input: &Value, _args: &CellPathOnlyArgs, span: Span) -> Value {
         ),
     }
 }
+
 fn int_from_string(a_string: &str, span: Span) -> Result<i64, ShellError> {
     // Get the Locale so we know what the thousands separator is
     let locale = get_system_locale();
@@ -148,29 +149,35 @@ fn int_from_string(a_string: &str, span: Span) -> Result<i64, ShellError> {
     let clean_string = no_comma_string.trim();
 
     // Hadle negative file size
-    if let Some(stripped_string) = clean_string.strip_prefix('-') {
-        match stripped_string.parse::<bytesize::ByteSize>() {
+    if let Some(stripped_negative_string) = clean_string.strip_prefix('-') {
+        match stripped_negative_string.parse::<bytesize::ByteSize>() {
             Ok(n) => Ok(-(n.as_u64() as i64)),
-            Err(_) => Err(ShellError::CantConvert {
-                to_type: "int".into(),
-                from_type: "string".into(),
-                span,
-                help: None,
-            }),
+            Err(_) => Err(string_convert_error(span)),
+        }
+    } else if let Some(stripped_positive_string) = clean_string.strip_prefix('+') {
+        match stripped_positive_string.parse::<bytesize::ByteSize>() {
+            Ok(n) if stripped_positive_string.starts_with(|c: char| c.is_ascii_digit()) => {
+                Ok(n.as_u64() as i64)
+            }
+            _ => Err(string_convert_error(span)),
         }
     } else {
         match clean_string.parse::<bytesize::ByteSize>() {
-            Ok(n) => Ok(n.0 as i64),
-            Err(_) => Err(ShellError::CantConvert {
-                to_type: "int".into(),
-                from_type: "string".into(),
-                span,
-                help: None,
-            }),
+            Ok(n) => Ok(n.as_u64() as i64),
+            Err(_) => Err(string_convert_error(span)),
         }
     }
 }
 
+fn string_convert_error(span: Span) -> ShellError {
+    ShellError::CantConvert {
+        to_type: "filesize".into(),
+        from_type: "string".into(),
+        span,
+        help: None,
+    }
+}
+
 #[cfg(test)]
 mod test {
     use super::*;

crates/nu-command/tests/commands/into_filesize.rs

@@ -63,8 +63,57 @@ fn into_filesize_negative_filesize() {
 }
 
 #[test]
-fn into_negative_filesize() {
+fn into_filesize_negative_str_filesize() {
+    let actual = nu!("'-3kib' | into filesize");
+
+    assert!(actual.out.contains("-3.0 KiB"));
+}
+
+#[test]
+fn into_filesize_wrong_negative_str_filesize() {
+    let actual = nu!("'--3kib' | into filesize");
+
+    assert!(actual.err.contains("can't convert string to filesize"));
+}
+
+#[test]
+fn into_filesize_negative_str() {
     let actual = nu!("'-1' | into filesize");
 
     assert!(actual.out.contains("-1 B"));
 }
+
+#[test]
+fn into_filesize_wrong_negative_str() {
+    let actual = nu!("'--1' | into filesize");
+
+    assert!(actual.err.contains("can't convert string to filesize"));
+}
+
+#[test]
+fn into_filesize_positive_str_filesize() {
+    let actual = nu!("'+1Kib' | into filesize");
+
+    assert!(actual.out.contains("1.0 KiB"));
+}
+
+#[test]
+fn into_filesize_wrong_positive_str_filesize() {
+    let actual = nu!("'++1Kib' | into filesize");
+
+    assert!(actual.err.contains("can't convert string to filesize"));
+}
+
+#[test]
+fn into_filesize_positive_str() {
+    let actual = nu!("'+1' | into filesize");
+
+    assert!(actual.out.contains("1 B"));
+}
+
+#[test]
+fn into_filesize_wrong_positive_str() {
+    let actual = nu!("'++1' | into filesize");
+
+    assert!(actual.err.contains("can't convert string to filesize"));
+}