Add generic type parameter to json() methods

[max-lt]

Proposal

Add an optional generic type parameter to json() methods on Response and Request for better TypeScript ergonomics.

Current signature

json(): Promise<unknown>;

Proposed signature

json<T = unknown>(): Promise<T>;

Benefits

• Allows response.json<MyType>() instead of (await response.json()) as MyType
• Default unknown maintains current strict behavior
• Fully backward compatible - existing code continues to work
• Consistent with other OpenWorkers APIs like KV.get<T>() and DB.query<T>()

Example

// Current (still works)
const data = (await response.json()) as { text: string };

// Proposed (new option)
const data = await response.json<{ text: string }>();

Affected types

• Response.json() (line 139 in fetch.d.ts)
• Request.json() (line 230 in fetch.d.ts)