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/migrator/src/migrations/m_2025_01_02/settings.rs

@@ -0,0 +1,62 @@
+use collections::HashMap;
+use std::{ops::Range, sync::LazyLock};
+use tree_sitter::{Query, QueryMatch};
+
+use crate::patterns::SETTINGS_NESTED_KEY_VALUE_PATTERN;
+use crate::MigrationPatterns;
+
+pub const SETTINGS_PATTERNS: MigrationPatterns = &[(
+    SETTINGS_NESTED_KEY_VALUE_PATTERN,
+    replace_deprecated_settings_values,
+)];
+
+fn replace_deprecated_settings_values(
+    contents: &str,
+    mat: &QueryMatch,
+    query: &Query,
+) -> Option<(Range<usize>, String)> {
+    let parent_object_capture_ix = query.capture_index_for_name("parent_key")?;
+    let parent_object_range = mat
+        .nodes_for_capture_index(parent_object_capture_ix)
+        .next()?
+        .byte_range();
+    let parent_object_name = contents.get(parent_object_range.clone())?;
+
+    let setting_name_ix = query.capture_index_for_name("setting_name")?;
+    let setting_name_range = mat
+        .nodes_for_capture_index(setting_name_ix)
+        .next()?
+        .byte_range();
+    let setting_name = contents.get(setting_name_range.clone())?;
+
+    let setting_value_ix = query.capture_index_for_name("setting_value")?;
+    let setting_value_range = mat
+        .nodes_for_capture_index(setting_value_ix)
+        .next()?
+        .byte_range();
+    let setting_value = contents.get(setting_value_range.clone())?;
+
+    UPDATED_SETTINGS
+        .get(&(parent_object_name, setting_name))
+        .and_then(|new_values| {
+            new_values
+                .iter()
+                .find_map(|(old_value, new_value)| {
+                    (*old_value == setting_value).then(|| new_value.to_string())
+                })
+                .map(|new_value| (setting_value_range, new_value))
+        })
+}
+
+static UPDATED_SETTINGS: LazyLock<HashMap<(&str, &str), Vec<(&str, &str)>>> = LazyLock::new(|| {
+    HashMap::from_iter([
+        (
+            ("chat_panel", "button"),
+            vec![("true", "\"always\""), ("false", "\"never\"")],
+        ),
+        (
+            ("scrollbar", "diagnostics"),
+            vec![("true", "\"all\""), ("false", "\"none\"")],
+        ),
+    ])
+});