[indexer] support conditional expressions

The indexer currently doesn't support conditionals, e.g.:

```
select: a, img
values: |
el.tagName === 'A'
? match(attribute(el, 'href'), 'pattern')
: match(attribute(el, 'src'), 'pattern')
```

which is supposed to act differently, depending on whether the element selected is an `a` or an `img` tag.

As a matter of fact, already the binary expression `el.tagName === 'A'` isn't supported, and the underlying problem goes deeper: the `select: a, img` expression will return an **array** of matching elements, and so every subsequent expression - binary and conditional - should also work on an array.

Start at the indexer expression evaluation entry point and trace how select, binary, and conditional expressions are handled. Reproduce the example with `select: a, img`, then verify that binary and conditional expressions work when the selection returns an array and produce the expected href or src match.

Written by the indexing model from the issue text.