A custom function or action of a library.
impl is set only for functions: it receives one map keyed by the param names and returns the result. Actions run in the host, never here.
impl
@type t() :: %OpenUI.Library.Callable{ description: String.t(), impl: (map() -> term()) | nil, name: String.t(), params: [OpenUI.Library.Prop.t()], returns: map() | nil, signature: String.t() }