introduce salsa and integrate into djls-project crate (#139)

crates/djls-project/src/db.rs

@@ -0,0 +1,31 @@
+use crate::meta::ProjectMetadata;
+
+#[salsa::db]
+pub trait Db: salsa::Database {
+    fn metadata(&self) -> &ProjectMetadata;
+}
+
+#[salsa::db]
+#[derive(Clone)]
+pub struct ProjectDatabase {
+    storage: salsa::Storage<ProjectDatabase>,
+    metadata: ProjectMetadata,
+}
+
+impl ProjectDatabase {
+    pub fn new(metadata: ProjectMetadata) -> Self {
+        let storage = salsa::Storage::new(None);
+
+        Self { storage, metadata }
+    }
+}
+
+#[salsa::db]
+impl Db for ProjectDatabase {
+    fn metadata(&self) -> &ProjectMetadata {
+        &self.metadata
+    }
+}
+
+#[salsa::db]
+impl salsa::Database for ProjectDatabase {}