Use serde 1.0.221 instead of serde_derive hackery (#38137)

serde 1.0.221 introduced serde_core into the build graph, which should
render explicitly depending on serde_derive for faster build times an
obsolote method.

Besides, I'm not even sure if that worked for us. My hunch is that at
least one of our deps would have `serde` with derive feature enabled..
and then, most of the crates using `serde_derive` explicitly were also
depending on gpui, which depended on `serde`.. thus, we wouldn't have
gained anything from explicit dep on `serde_derive`

Release Notes:

- N/A

crates/theme_importer/src/vscode/theme.rs

@@ -6,12 +6,32 @@ use crate::vscode::VsCodeTokenColor;
 #[derive(Deserialize, Debug)]
 pub struct VsCodeTheme {
     #[serde(rename = "$schema")]
+    #[expect(
+        unused,
+        reason = "This field was found to be unused with serde library bump; it's left as is due to insufficient context on PO's side, but it *may* be fine to remove"
+    )]
     pub schema: Option<String>,
     pub name: Option<String>,
+    #[expect(
+        unused,
+        reason = "This field was found to be unused with serde library bump; it's left as is due to insufficient context on PO's side, but it *may* be fine to remove"
+    )]
     pub author: Option<String>,
+    #[expect(
+        unused,
+        reason = "This field was found to be unused with serde library bump; it's left as is due to insufficient context on PO's side, but it *may* be fine to remove"
+    )]
     pub maintainers: Option<Vec<String>>,
     #[serde(rename = "semanticClass")]
+    #[expect(
+        unused,
+        reason = "This field was found to be unused with serde library bump; it's left as is due to insufficient context on PO's side, but it *may* be fine to remove"
+    )]
     pub semantic_class: Option<String>,
+    #[expect(
+        unused,
+        reason = "This field was found to be unused with serde library bump; it's left as is due to insufficient context on PO's side, but it *may* be fine to remove"
+    )]
     #[serde(rename = "semanticHighlighting")]
     pub semantic_highlighting: Option<bool>,
     pub colors: Colors,