stop the prompt from removing the last newline (#14590)

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# Description
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I tried to setup a multiline prompt like this.
<img width="175" alt="スクリーンショット 2024-12-15 17 45 06"
src="https://github.com/user-attachments/assets/8d00a203-b341-45ce-8427-b4d5a9d3d7c3"
/>

But when I set PROMT_COMMAND like this, 

```nu
$env.PROMPT_COMMAND = {|| $"(ansi reset)(ansi magenta)(date now | format date "%Y-%m-%dT%H:%M:%S%z")\n(pwd)\n" } 
```

The result is like this, due to dropping `\n` and `\r` on
`prompt_update.rs`.
<img width="185" alt="スクリーンショット 2024-12-15 17 54 21"
src="https://github.com/user-attachments/assets/5ead998e-6f87-479f-b2de-e267f0cc3acd"
/>

Currently, adding two newlines can detour the drop.
I think this drop newline makes little sense, so I removed it on this
PR.
If you don't like it, feel free to close it.

# User-Facing Changes
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helps us keep track of breaking changes. -->

Trailing newline of PROMPT_COMMAND is not dropped anymore.

# Tests + Formatting
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This is a subtle change just on prompt string, so I think particular
test is not so necessary.

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As far as I read
https://www.nushell.sh/book/coloring_and_theming.html#prompt-configuration-and-coloring
, the behavior seems undocumented.

crates/nu-cli/src/prompt_update.rs

@@ -87,14 +87,6 @@ fn get_prompt_string(
                     x.insert_str(0, "\x1b[0m")
                 };
 
-                // Just remove the very last newline.
-                if x.ends_with('\n') {
-                    x.pop();
-                }
-
-                if x.ends_with('\r') {
-                    x.pop();
-                }
                 x
             });
             // Let's keep this for debugging purposes with nu --log-level warn