…king a tag closer

This commit marks the start of a bookmark one byte before
the tag name start for tag openers, and two bytes before
the tag name for tag closers.

Setting a bookmark on a tag should set its "start" position
before the opening "<", e.g.:

```
<div> Testing a <b>Bookmark</b>
----------------^
```

The current calculation assumes this is always one byte
to the left from $tag_name_starts_at.

However, in tag closers that index points to a solidus
symbol "/":

```
<div> Testing a <b>Bookmark</b>
----------------------------^
```

The bookmark should therefore start two bytes before the tag name:

```
<div> Testing a <b>Bookmark</b>
---------------------------^
```

…closers' into wp_html_processor