migrator: Add migration for settings changed prior to migrator-introduction (#27375)

This PR updates two existing settings to use the settings migrator
instead of a manually implemented visitor. Both of these settings were
changed prior to the introduction of automatic migrations and the
visitor ensured that the settings were kept backwards compatible. See
https://github.com/zed-industries/zed/pull/22200 and
https://github.com/zed-industries/zed/pull/22364 respectively.

WIth this change, existing user configurations are updated accordingly
and the corresponding settings can derive `Deserialize` again.

I also added tests for the replacement of settings values, as there was
no test for this behaviour. Additionally, I added a seperate test for
the existing migration of `always_show_close_button`, since that
migration updated both the key and value.

Release Notes:

- N/A

crates/editor/src/editor_settings.rs

@@ -156,7 +156,7 @@ pub struct ScrollbarAxes {
 /// Which diagnostic indicators to show in the scrollbar.
 ///
 /// Default: all
-#[derive(Copy, Clone, Debug, Serialize, JsonSchema, PartialEq, Eq)]
+#[derive(Copy, Clone, Debug, Serialize, Deserialize, JsonSchema, PartialEq, Eq)]
 #[serde(rename_all = "lowercase")]
 pub enum ScrollbarDiagnostics {
     /// Show all diagnostic levels: hint, information, warnings, error.
@@ -171,55 +171,6 @@ pub enum ScrollbarDiagnostics {
     None,
 }
 
-impl<'de> Deserialize<'de> for ScrollbarDiagnostics {
-    fn deserialize<D>(deserializer: D) -> Result<Self, D::Error>
-    where
-        D: serde::Deserializer<'de>,
-    {
-        struct Visitor;
-
-        impl serde::de::Visitor<'_> for Visitor {
-            type Value = ScrollbarDiagnostics;
-
-            fn expecting(&self, f: &mut std::fmt::Formatter) -> std::fmt::Result {
-                write!(
-                    f,
-                    r#"a boolean or one of "all", "information", "warning", "error", "none""#
-                )
-            }
-
-            fn visit_bool<E>(self, b: bool) -> Result<Self::Value, E>
-            where
-                E: serde::de::Error,
-            {
-                match b {
-                    false => Ok(ScrollbarDiagnostics::None),
-                    true => Ok(ScrollbarDiagnostics::All),
-                }
-            }
-
-            fn visit_str<E>(self, s: &str) -> Result<Self::Value, E>
-            where
-                E: serde::de::Error,
-            {
-                match s {
-                    "all" => Ok(ScrollbarDiagnostics::All),
-                    "information" => Ok(ScrollbarDiagnostics::Information),
-                    "warning" => Ok(ScrollbarDiagnostics::Warning),
-                    "error" => Ok(ScrollbarDiagnostics::Error),
-                    "none" => Ok(ScrollbarDiagnostics::None),
-                    _ => Err(E::unknown_variant(
-                        s,
-                        &["all", "information", "warning", "error", "none"],
-                    )),
-                }
-            }
-        }
-
-        deserializer.deserialize_any(Visitor)
-    }
-}
-
 /// The key to use for adding multiple cursors
 ///
 /// Default: alt