feat: add goose env command

[mfridman]

This PR moves all env config-related bits to an internal cfg package (inspired by the go command), and exposes a new command to see the available and set env variables.

Example from the root of this repo set by direnv:

$ goose env
GOOSE_DRIVER="postgres"
GOOSE_DBSTRING="postgresql://dbuser:password1@localhost:5433/testdb?sslmode=disable"
GOOSE_MIGRATION_DIR="./tests/e2e/testdata/postgres/migrations"
NO_COLOR="false"

And when not set, here are the defaults:

$ goose env
GOOSE_DRIVER=""
GOOSE_DBSTRING=""
GOOSE_MIGRATION_DIR="."
NO_COLOR="false"

Closes #419