[Website] Flatten the stored runtime configuration format

[adamziel]

What does this PR change?

Flattens the RuntimeConfiguration type used by the Playground website from the previous, nested shape:

{
	preferredVersions?: {
		php: SupportedPHPVersion | 'latest';
		wp: string | 'latest';
	};
	features?: {
		intl?: boolean;
		networking?: boolean;
	};
	extraLibraries?: ExtraLibrary[];
	constants?: PHPConstants;
}

to

export interface RuntimeConfiguration {
	phpVersion: SupportedPHPVersion;
	wpVersion: string;
	intl: boolean;
	networking: boolean;
	extraLibraries: ExtraLibrary[];
	constants: PHPConstants;
}

Why?

The old configuration format is a subset of Blueprint v1 type. Since we're adding support for Blueprints v2, we need a format that will be convenient regardless of the underlying Blueprint version. The nesting made interop more difficult, hence I've flattened the data structure.

A part of #2586.

Testing Instructions (or ideally a Blueprint)

CI