Add Pager helper with info about where the value comes from

In preparation of fixing issue #1063.
This is a pure refactoring with no intended functional side effects.

src/lib.rs

@@ -31,6 +31,8 @@ mod less;
 pub mod line_range;
 mod output;
 #[cfg(feature = "paging")]
+mod pager;
+#[cfg(feature = "paging")]
 pub(crate) mod paging;
 mod preprocessor;
 mod pretty_printer;