* feat(logger): implement slog.Handler

This implements slog.Handler interface. You can use Log as an slog
handler.

* feat: compile for go1.20 and go1.21

* refactor: match log/slog level values

* feat: add slog tests

* chore: downgrade the minimum go version to go1.19

* doc: add slog support

* refactor: rename files

* fix: add emoji test

* chore!: change default timestamp and level key names

* fix: lint