d2ir: Single Level Ampersand Filters

[nhooyr]

Depends on #1479 being merged.

I didn't implement recursive filters in this PR. i.e you can't do something like:

*: {
  &style: {
    fill: red
    multiple: true
  }
  label: red and multiple
}

You have to do the following instead:

*: {
  style: {
    &fill: red
    &multiple: true
  }
  label: red and multiple
}

[nhooyr]

In fact I don't think we want to have composite filters anyhow. I think single field filters are easier to read and much easier to implement. For example for composite filters we'd have to error if the filter itself had ampersand filters because that wouldn't make sense.