Highlights:
  * Plush templates
  * .env files
  * --env staging

Details:

To make use of Plush templates, rename `fly.toml` to `fly.toml.plush`.
This prevents existing `fly.toml` files from being interpreted as
templates, and enables additional. templating formats to be added later.

We can provide predefined variables and functions to be referenced
by templates.  Additionally, variables are loaded from `.env` files,
the OS environment, and `--env` flags.

More information on Plush can be found at:

https://github.com/gobuffalo/plush?tab=readme-ov-file#plush

---

Previously `--env` or `-e` is for environment variables, and required a syntax
of name=value.  If only a name is provided, treat this as if `FLY_ENV=name` was
specified instead.  Note that `FLY_ENV` can also be set as an environment
variable.

If `FLY_ENV` is set, `fly-${FLY_ENV}.toml` and `.env.${FLY_ENV}` will be used.