cli: Use latest specified flag value when repeated (#15919)

# Description

This PR makes the last specified CLI arguments take precedence over the
earlier ones.

Existing command line tools that align with the new behaviour include:
- `neovim`: `nvim -u a.lua -u b.lua` will use `b.lua`
- `ripgrep`: you can have `--smart-case` in your user config but
override it later with `--case-sensitive` or `--ignore-case` (not
exactly the same flag override as the one I'm talking about but I think
it's still a valid example of latter flags taking precedence over the
first ones)

I think a flag defined last can be considered an override. This allows
having a `nu` alias that includes some default config (`alias nu="nu
--config something.nu"`) but being able to override that default config
as if using `nu` normally.
 
## Example

```sh
nu --config config1.nu --config config2.nu -c '$nu.config-path'
```
The current behavior would print `config1.nu`, and the new one would
print `config2.nu`

## Implementation

Just `.rev()` the iterator to search for arguments starting from the end
of the list. To support that I had to modify the return type of
`named_iter` (I couldn't find a more generic way than
`DoubleEndedIterator`).

# User-Facing Changes

- Users passing repeated flags and relying in nushell using the first
value will experience breakage. Given that right now there's no point in
passing a flag multiple times I guess not many users will be affected

# Tests + Formatting

I added a test that checks the new behavior with `--config` and
`--env-config`. I'm happy to add more cases if needed

# After Submitting

crates/nu-protocol/src/ast/call.rs

@@ -128,7 +128,8 @@ impl Call {
 
     pub fn named_iter(
         &self,
-    ) -> impl Iterator<Item = &(Spanned<String>, Option<Spanned<String>>, Option<Expression>)> {
+    ) -> impl DoubleEndedIterator<Item = &(Spanned<String>, Option<Spanned<String>>, Option<Expression>)>
+    {
         self.arguments.iter().filter_map(|arg| match arg {
             Argument::Named(named) => Some(named),
             Argument::Positional(_) => None,
@@ -222,7 +223,7 @@ impl Call {
     }
 
     pub fn get_flag_expr(&self, flag_name: &str) -> Option<&Expression> {
-        for name in self.named_iter() {
+        for name in self.named_iter().rev() {
             if flag_name == name.0.item {
                 return name.2.as_ref();
             }
@@ -232,7 +233,7 @@ impl Call {
     }
 
     pub fn get_named_arg(&self, flag_name: &str) -> Option<Spanned<String>> {
-        for name in self.named_iter() {
+        for name in self.named_iter().rev() {
             if flag_name == name.0.item {
                 return Some(name.0.clone());
             }