linux: Make prompt detail selectable (#21405)

Closes #21305

As Linux doesn’t have native prompts, Zed uses a custom GPU-based
prompt, like the "About Zed" prompt. Currently, the detail in the prompt
isn’t selectable.

This PR fixes that by using the editor's multi-line selectable
functionality to make the detail selectable (and thus copyable). It
achieves this by disabling editing and setting the cursor to
transparent. The editor also does all the heavy lifting, like
double-clicking to select a word or triple-clicking to select a line,
like what user expects from selectable.

Before/After:

<img
src="https://github.com/user-attachments/assets/2012a6cc-a1ed-4efe-8bfb-440a9259f07a"
alt="before" width="360px" />

<img
src="https://github.com/user-attachments/assets/31922ef5-cb2d-4e90-a1a1-00843e767432"
alt="after" width="360px" />

When detail is `None` or empty string:

<img
src="https://github.com/user-attachments/assets/2be5c921-bda1-4db3-85cd-b4b0e2df86d2"
alt="none" width="360px" />

Release Notes:

- N/A

crates/zed/Cargo.toml

@@ -69,6 +69,7 @@ language_tools.workspace = true
 languages = { workspace = true, features = ["load-grammars"] }
 libc.workspace = true
 log.workspace = true
+markdown.workspace = true
 markdown_preview.workspace = true
 menu.workspace = true
 mimalloc = { version = "0.1", optional = true }