Restore tilde expansion on external command names (#13001)

# Description

Fix a regression introduced by #12921, where tilde expansion was no
longer done on the external command name, breaking things like

```nushell
> ~/.cargo/bin/exa
```

This properly handles quoted strings, so they don't expand:

```nushell
> ^"~/.cargo/bin/exa"
Error: nu:🐚:external_command

  × External command failed
   ╭─[entry #1:1:2]
 1 │ ^"~/.cargo/bin/exa"
   ·  ─────────┬────────
   ·           ╰── Command `~/.cargo/bin/exa` not found
   ╰────
  help: `~/.cargo/bin/exa` is neither a Nushell built-in or a known external command

```

This required a change to the parser, so the command name is also parsed
in the same way the arguments are - i.e. the quotes on the outside
remain in the expression. Hopefully that doesn't break anything else. 🤞

Fixes #13000. Should include in patch release 0.94.1

cc @YizhePKU

# User-Facing Changes
- Tilde expansion now works again for external commands
- The `command` of `run-external` will now have its quotes removed like
the other arguments if it is a literal string
- The parser is changed to include quotes in the command expression of
`ExternalCall` if they were present

# Tests + Formatting
I would like to add a regression test for this, but it's complicated
because we need a well-known binary within the home directory, which
just isn't a thing. We could drop one there, but that's kind of a bad
behavior for a test to do. I also considered changing the home directory
for the test, but that's so platform-specific - potentially could get it
working on specific platforms though. Changing `HOME` env on Linux
definitely works as far as tilde expansion works.

- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

crates/nu-parser/src/parser.rs

@@ -278,13 +278,14 @@ pub fn parse_external_call(working_set: &mut StateWorkingSet, spans: &[Span]) ->
         let arg = parse_expression(working_set, &[head_span]);
         Box::new(arg)
     } else {
-        let (contents, err) = unescape_unquote_string(&head_contents, head_span);
+        // Eval stage will unquote the string, so we don't bother with that here
+        let (contents, err) = unescape_string(&head_contents, head_span);
         if let Some(err) = err {
             working_set.error(err)
         }
 
         Box::new(Expression {
-            expr: Expr::String(contents),
+            expr: Expr::String(String::from_utf8_lossy(&contents).into_owned()),
             span: head_span,
             ty: Type::String,
             custom_completion: None,