Add blanket `impl<T: Write> OutputSink for T`

Currently, `HtmlRewriter::try_new` takes _only_ closures and nothing else. For the common case of 'I have a buffer to write to' this causes the code to be more complicated than necessary:

```rust
let mut output = vec![];

// NOTE: never panics because encoding is always "utf-8".
let mut rewriter = HtmlRewriter::try_new(settings.into(), |c: &[u8]| {
output.extend_from_slice(c);
})
.unwrap();
```

If `OutputSink` took any `io::Write` impl, it could be simpler:

```rust
let mut output = vec![];

// NOTE: never panics because encoding is always "utf-8".
let mut rewriter = HtmlRewriter::try_new(settings.into(), &mut output).unwrap();
```

Start at OutputSink and HtmlRewriter::try_new, then trace how the current closure sink is accepted. Check the existing output-handling examples or tests; done means a mutable io::Write target can be passed directly while the current closure-based usage remains supported.

Written by the indexing model from the issue text.