Add a new Pager type that wraps output to an external pager program when
stdout is a TTY, following the same conventions as git. The pager
command is resolved from $PAGER, falling back to `less`. An empty $PAGER
disables paging. If the pager fails to spawn, we fall back to stdout.
Previously, +explain-config wrote directly to stdout with no paging,
which meant long help text would scroll by. Now output is automatically
piped through the user's preferred pager when running interactively. A
--no-pager flag is available to disable this.
