assistant: Normalize line endings for prompts loaded from the prompt library (#15708)

This PR makes it so we normalize the line endings for prompts to LFs
(`\n`) when we load a prompt from the library.

In some cases, prompts could end up with CRLF (`\r\n`) line endings.
When these prompts were used with the `/prompt` slash command and
summarily run, the prompt text would be converted into a rope, causing
the line endings to be normalized to LFs.

However, this would happen _after_ the ranges for the
`SlashCommandOutputSection`s were computed based on the text that still
contained the CRLFs. This would then cause these ranges to be invalid
for the text with the normalized endings, resulting in a panic when
converting them to anchors.

Fixes https://github.com/zed-industries/zed/issues/15652.

Release Notes:

- N/A

Co-authored-by: Max <max@zed.dev>

crates/assistant_slash_command/src/assistant_slash_command.rs

@@ -60,7 +60,7 @@ pub type RenderFoldPlaceholder = Arc<
         + Fn(ElementId, Arc<dyn Fn(&mut WindowContext)>, &mut WindowContext) -> AnyElement,
 >;
 
-#[derive(Default)]
+#[derive(Debug, Default)]
 pub struct SlashCommandOutput {
     pub text: String,
     pub sections: Vec<SlashCommandOutputSection<usize>>,