Wrap extension schema version in a newtype (#9872)

This PR wraps the extension schema version in a newtype, for some
additional type safety.

Release Notes:

- N/A

crates/extension/src/extension_manifest.rs

@@ -11,7 +11,7 @@ use std::{
 use util::SemanticVersion;
 
 /// This is the old version of the extension manifest, from when it was `extension.json`.
-#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+#[derive(Debug, PartialEq, Eq, Clone, Serialize, Deserialize)]
 pub struct OldExtensionManifest {
     pub name: String,
     pub version: Arc<str>,
@@ -31,12 +31,23 @@ pub struct OldExtensionManifest {
     pub grammars: BTreeMap<Arc<str>, PathBuf>,
 }
 
-#[derive(Clone, PartialEq, Eq, Debug, Deserialize, Serialize)]
+#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Hash, Clone, Copy, Serialize, Deserialize)]
+pub struct SchemaVersion(pub i32);
+
+impl SchemaVersion {
+    pub const ZERO: Self = Self(0);
+
+    pub fn is_v0(&self) -> bool {
+        self == &Self::ZERO
+    }
+}
+
+#[derive(Debug, PartialEq, Eq, Clone, Serialize, Deserialize)]
 pub struct ExtensionManifest {
     pub id: Arc<str>,
     pub name: String,
     pub version: Arc<str>,
-    pub schema_version: i32,
+    pub schema_version: SchemaVersion,
 
     #[serde(default)]
     pub description: Option<String>,
@@ -122,7 +133,7 @@ fn manifest_from_old_manifest(
         description: manifest_json.description,
         repository: manifest_json.repository,
         authors: manifest_json.authors,
-        schema_version: 0,
+        schema_version: SchemaVersion::ZERO,
         lib: Default::default(),
         themes: {
             let mut themes = manifest_json.themes.into_values().collect::<Vec<_>>();