CI use 6.1 nightlies now that Swift development is happening in the 6.1
branch

### Motivation

We recently demoed [building a streaming ChatGPT proxy with Swift
OpenAPI, at FOSDEM 2025][0]. This PR adds the resulting package for
people to refer to as reference material.

### Modifications

- Add package reflecting final state of the presentation.
- Add a `.env.example` file.
- Add covering README with link to presentation, abstract, and usage. 
- Add VS Code settings for Dev Containers.

### Result

Reference project available for people watching the presentation.

### Test Plan

CI will build the example.

[0]:
https://fosdem.org/2025/schedule/event/fosdem-2025-5230-live-coding-a-streaming-chatgpt-proxy-with-swift-openapi-from-scratch-/

### Motivation

This project has moved into the Incubating stage of the SSWG incubation
process:
https://www.swift.org/sswg/incubated-packages.html#incubating-projects

### Modifications

Updated the README badge to reflect that.

### Result

Accurate badge.

### Test Plan

Previewed locally.

Limit this to Apple platforms except macOS in order to unblock use of
swift-openapi-generator on more platforms like Windows, Android,
FreeBSD, and so on.

Rename nightly_6_1 params to nightly_next; see
apple/swift-nio#3122

Only apply standard swift settings on valid targets. The current check
ignores plugins but that is not comprehensive enough.