Add visionOS platform support (#85)

### Motivation

While this isn't technically necessary, as all versions of a platform
not explicitly mentioned are assumed to be supported, it's better to be
explicit here.

### Modifications

Add `visionOS(.v1)` to the list of supported platforms.

### Result

Clearer support matrix.

### Test Plan

N/A, this is basically just a documentation change.

Author: czechboy0

Package.swift

@@ -24,7 +24,7 @@ let swiftSettings: [SwiftSetting] = [
 let package = Package(
     name: "swift-openapi-runtime",
     platforms: [
-        .macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .watchOS(.v6),
+        .macOS(.v10_15), .iOS(.v13), .tvOS(.v13), .watchOS(.v6), .visionOS(.v1)
     ],
     products: [
         .library(