fix: Include exercises folder in the project structure behind a feature (#917)

closes #859
closes #913
closes #942

Cargo.toml

@@ -11,7 +11,7 @@ console = "0.7.7"
 notify = "4.0.15"
 toml = "0.4.10"
 regex = "1.1.6"
-serde = {version = "1.0.10", features = ["derive"]}
+serde = { version = "1.0.10", features = ["derive"] }
 
 [[bin]]
 name = "rustlings"
@@ -21,3 +21,6 @@ path = "src/main.rs"
 assert_cmd = "0.11.0"
 predicates = "1.0.1"
 glob = "0.3.0"
+
+[features]
+exercises = []