[RFC] Strict Mode

Add a strict mode to version 3.x
To be the default to True on 4.x

What this will do:

- Limit exposure of environment variables to the string interpolation (jinja, format, get)
- Expose only the specified list of vars

```
Dynaconf(
env={
"FOOBAR":"FOOBAR",
"THING": "ANOTHERNAME"
}
)
```

No files, tests, or entry points are named. Start by locating the environment-variable exposure and string-interpolation paths for jinja, format, and get, then review how the env mapping is configured. Done should include a defined strict-mode behavior that exposes only the specified variables, with coverage for the planned v3.x mode and the v4.x default.

Written by the indexing model from the issue text.