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/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"));
+}