Making nushell works better with external args which surrounded by backtick quotes (#13910)

# Description
Fixes: #13431
Fixes: #13578

The issue happened because nushell thinks external program name and
external arg with totally same rule. But actually they are a little bit
different.
When parsing external program name, backtick is a thing and it should be
keeped.
But when parsing external args, backtick is just a mark that it's a
**bareword which may contain space**. So in this context, it's already
useless.

# User-Facing Changes
After the pr, the following command will work as intended.
```nushell
> ^echo `"hello"`
hello
```

# Tests + Formatting
Added 3 test cases.

crates/nu-parser/src/parser.rs

@@ -240,10 +240,26 @@ fn parse_unknown_arg(
 /// string, where each balanced pair of quotes is parsed as a separate part of the string, and then
 /// concatenated together.
 ///
+/// `keep_surround_backtick_quote` should be true when parsing it as command name.  Or else it
+/// should be false.
+///
 /// For example, `-foo="bar\nbaz"` becomes `$"-foo=bar\nbaz"`
-fn parse_external_string(working_set: &mut StateWorkingSet, span: Span) -> Expression {
-    let contents = &working_set.get_span_contents(span);
+fn parse_external_string(
+    working_set: &mut StateWorkingSet,
+    mut span: Span,
+    keep_surround_bakctick_quote: bool,
+) -> Expression {
+    let mut contents = working_set.get_span_contents(span);
 
+    if !keep_surround_bakctick_quote
+        && contents.len() > 1
+        && contents.starts_with(b"`")
+        && contents.ends_with(b"`")
+    {
+        contents = &contents[1..contents.len() - 1];
+        // backtick quote is useless in this case, so span is required to updated.
+        span = Span::new(span.start + 1, span.end - 1);
+    }
     if contents.starts_with(b"r#") {
         parse_raw_string(working_set, span)
     } else if contents
@@ -441,7 +457,7 @@ fn parse_regular_external_arg(working_set: &mut StateWorkingSet, span: Span) ->
     } else if contents.starts_with(b"[") {
         parse_list_expression(working_set, span, &SyntaxShape::Any)
     } else {
-        parse_external_string(working_set, span)
+        parse_external_string(working_set, span, false)
     }
 }
 
@@ -463,7 +479,7 @@ pub fn parse_external_call(working_set: &mut StateWorkingSet, spans: &[Span]) ->
         let arg = parse_expression(working_set, &[head_span]);
         Box::new(arg)
     } else {
-        Box::new(parse_external_string(working_set, head_span))
+        Box::new(parse_external_string(working_set, head_span, true))
     };
 
     let args = spans[1..]