fix: CustomEvent generic type

[Cherry]

Fixes #1504

This overrides the CustomEvent types to better match other implementations.

This is my first time contributing to workerd, and really using JSG at all, so apologies if there's anything weird with this change. If there's a better way to do this, please just let me know.

Old output:

export declare class CustomEvent extends Event {
  constructor(type: string, init?: CustomEventCustomEventInit);
  get detail(): any | undefined;
}

New output:

export declare class CustomEvent<T = any> extends Event {
  constructor(type: string, init?: CustomEventCustomEventInit);
  get detail(): T;
}

TypeScript lib.dom.d.ts reference: https://github.com/microsoft/TypeScript/blob/1d6d962d3132a901f5fb48be4a389c45faf5a74e/src/lib/dom.generated.d.ts#L5935

[github-actions]

CLA Assistant Lite bot All contributors have signed the CLA ✍️ ✅

[Cherry]

I have read the CLA Document and I hereby sign the CLA

[Cherry]

Test failure here looks to be unrelated:

LLVM ERROR: IO failure on output stream: no space on device

[jasnell]

Yep, don't worry about that. it's a known issue with the windows-debug job

[mrbbot]

Apologies for commenting after this was merged, but I think this could be simplified to:

Suggested change

	JSG_TS_OVERRIDE(<T = any> extends Event {
	constructor(type: string, init?: CustomEventCustomEventInit);
	get detail(): T;
	});
	JSG_TS_OVERRIDE(<T = any> {
	get detail(): T;
	});

[Cherry]

Can confirm that produces the same types output:

export class CustomEvent<T = any> extends Event {
  constructor(type: string, init?: CustomEventCustomEventInit);
  get detail(): T;
}

Opened a quick PR to improve: #1766

Thanks for catching!