Support built-in Zed prompts for all platforms (#26201)

This pull request does two things:

1. Adds a setting to force Zed to use the built-in prompts, instead of
the system provided ones. I've personally found the system prompts on
macOS often fail to respond to keyboard input, are slow to render
initially, and don't match Zed's style.
2. Makes the previously Linux-only Zed provided prompts available to
everybody using the above setting.

Release Notes:
- Added support for a built-in prompting system, regardless of platform.
Use the new `use_system_prompts` setting to control whether to use the
system provided prompts or Zed's built-in system. Note that on Linux,
this setting has no effect, as Linux doesn't have a system prompting
mechanism.

crates/zed/Cargo.toml

@@ -75,7 +75,6 @@ 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
 migrator.workspace = true
@@ -125,6 +124,7 @@ theme_selector.workspace = true
 time.workspace = true
 toolchain_selector.workspace = true
 ui.workspace = true
+ui_prompt.workspace = true
 url.workspace = true
 urlencoding.workspace = true
 util.workspace = true