debugger: Remove fake adapter and un-gate GDB (#27557)

This is a clean-up PR in anticipation of introduction of Debugger
Registry. I wanna get rid of DebugAdapterKind (or rather, it being an
enum).
Release Notes:

- N/A

---------

Co-authored-by: Anthony Eid <hello@anthonyeid.me>
Co-authored-by: Anthony <anthony@zed.dev>

crates/dap/src/dap.rs

@@ -0,0 +1,17 @@
+pub mod adapters;
+pub mod client;
+pub mod debugger_settings;
+pub mod proto_conversions;
+mod registry;
+pub mod transport;
+
+pub use dap_types::*;
+pub use registry::DapRegistry;
+pub use task::{DebugAdapterConfig, DebugRequestType};
+
+pub type ScopeId = u64;
+pub type VariableReference = u64;
+pub type StackFrameId = u64;
+
+#[cfg(any(test, feature = "test-support"))]
+pub use adapters::FakeAdapter;