Adds a `ShowDebugOverlay` key assignment that will create a tab
overlay that shows a limited number of recently logged events.
refs: https://github.com/wez/wezterm/issues/641
80214319ae broke the use of RUST_LOG to
turn up trace logging.
This commit refactors logger initialization into the env-bootstrap crate
so that it is centralized, and adopts the use of `WEZTERM_LOG` to
override the default logging filters, rather than `RUST_LOG`.
