Allow using `uv venv` with `--format=json`

[InSyncWithFoo]

This is what uv venv currently outputs on Windows:

Using Python 3.12.4 interpreter at: ...\python.exe
Creating virtualenv at: .venv
Activate with: .venv\Scripts\activate

It is good for human consumption, but not so much for machines. I propose that uv venv outputs a JSON object in the following format when used with --format=json:

interface VenvOutput {
  python: string;
  venv: string;
  activateCommand: string;
}

Example output:

{
  "python": "...\\python.exe",
  "venv": ".venv",
  "activateCommand": ".venv\\Scripts\\activate"
}

This is a subtask of #3199.

[zanieb]

Part of #411