[Feature]: Allow specifying output file name

### 🚀 Feature Request

I'd like to be able to specify my own name for the output file. Anything other than index.html.
Opening because #17407 was closed without resolution, and I'd really like this feature.
If it's approved, I'll implement it. I've pretty much got it written already.

### Example

_No response_

### Motivation

Improved flexibility, and a feature that's very common in similar software, so users expect it. I certainly did.

No file, test, or entry point is named in the issue. Start by reading the prior discussion in #17407 and tracing where Playwright chooses the output filename; done means users can configure a non-index.html name and the behavior is covered by appropriate tests.

Written by the indexing model from the issue text.