pager: set some environment variables if they're not set
Git did this already [1] [2]. We want this behavior too [3].
This provides a better default user experience (like, supporting colors) if
users have things like "PAGER=less" set, which is not uncommon.
The environment variables are provided by a method so extensions can
override them on demand.
[1]: https://github.com/git/git/blob/
6a5ff7acb5965718cc7016c0ab6c601454fd7cde/pager.c#L87
[2]: https://github.com/git/git/blob/
6a5ff7acb5965718cc7016c0ab6c601454fd7cde/Makefile#L1545
[3]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/094780.html