This pull request adds a new `max_errors` argument to `parse_option_T`
and `Herb.parse` so that the parser stops generating errors after the
limit is reached. It defaults to 25 but can be overwritten by passing
the `max_errors` keyword argument to `Herb.parse`:

```ruby
Herb.parse(source, max_errors: 50)
```

Setting `max_errors` to `nil` disables the limit.

This prevents pathological inputs (e.g. thousands of unclosed tags) from
generating thousands of error objects, which wastes memory and produces
output no one will read.

Follow-up to #1795.