Support serde serialization and deserialization

This commit adds serialization and deserialization support for the
Verbosity type. The verbosity is serialized using the log::LevelFilter
enum that represents the equivalent number of verbose and quiet flags.

The serialized value is the uppercase variant of the enum variant.
Deserialing is case-insensitive.

Fixes: clap-rs#88

Cargo.toml

@@ -112,13 +112,19 @@ pre-release-replacements = [
 [badges]
 codecov = { repository = "clap-rs/clap-verbosity-flag" }
 
+[features]
+serde = ["dep:serde", "log/serde"]
+
 [dependencies]
 log = "0.4.1"
 clap = { version = "4.0.0", default-features = false, features = ["std", "derive"] }
+serde = { version = "1.0.210", features = ["derive"], optional = true}
 
 [dev-dependencies]
 clap = { version = "4.5.4", default-features = false, features = ["help", "usage"] }
 env_logger = "0.11.3"
+serde_test = { version = "1.0.177" }
+toml = { version = "0.8.19" }
 tracing = "0.1"
 tracing-subscriber = "0.3"
 tracing-log = "0.2"