derive Arbitrary to generate better random inputs on fuzzing

fuzz/Cargo.toml

@@ -10,7 +10,7 @@ cargo-fuzz = true
 
 [dependencies]
 libfuzzer-sys = "0.4"
-arbitrary = "1"
+arbitrary = { version = "1", features = ["derive"] }
 
 [dependencies.tui-textarea]
 path = ".."