refactor(scrollbar): experimental prefix over feature

A feature flag has quite some complexity with it and is annyoing.
Write a rustdoc comment about it and be clear about it being experimental and include it as is.

Cargo.toml

@@ -11,14 +11,6 @@ categories = ["command-line-interface"]
 include = ["src/**/*", "README.md"]
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
-[features]
-default = []
-experimental_scrollbar = []
-
-[[example]]
-name = "example"
-required-features = ["experimental_scrollbar"]
-
 [dependencies]
 ratatui = { version = "0.26", default-features = false }
 unicode-width = "0.1"