Fix some bugs in keymap handling (#3895)

- `base_keymap` setting was not respected, now it is
- without a `~/.config/zed/keymap.json` file, we would fail to load the
*default* keymap

Co-authored-by: Marshall <marshall@zed.dev>

crates/command_palette/src/command_palette.rs

@@ -370,6 +370,7 @@ mod tests {
     use gpui::TestAppContext;
     use language::Point;
     use project::Project;
+    use settings::KeymapFile;
     use workspace::{AppState, Workspace};
 
     #[test]
@@ -503,7 +504,20 @@ mod tests {
             workspace::init(app_state.clone(), cx);
             init(cx);
             Project::init_settings(cx);
-            settings::load_default_keymap(cx);
+            KeymapFile::parse(
+                r#"[
+                    {
+                        "bindings": {
+                            "cmd-n": "workspace::NewFile",
+                            "enter": "menu::Confirm",
+                            "cmd-shift-p": "command_palette::Toggle"
+                        }
+                    }
+                ]"#,
+            )
+            .unwrap()
+            .add_to_cx(cx)
+            .unwrap();
             app_state
         })
     }