If the typo itself contains magic pattern characters we can't reliable find
it in the text.

An iterator in lua stops if it returns nil.
This change will get a new typo from the unfiltered_iterator if
we encounter an empty one instead of returning nil.

apparently in the typo list produced by aspell there is a blank line
which messes with our search for typos.

Before this change we assumed that when we are at the beginning of the
text (index == 1) and don't find the typo is must be the beginning of the
text but it can also be the end of the text.
We now also check the end if the beginning was not what we were lookig for.

This activates spellchecking for most "text focused" languages like
latex or markdown.

This happens because the "default" token gets emitted for each not matched
char resulting in lots of small "default" tokens.

This markdown:

> typu

will result in the token stream
{"default", 2, "default", 3, "default", 4, "default", 5}
and a typo stream with a single entry spanning all of the tokens
{{"typu", 1, 5}}.

Not checking if the typo is longer then the current token leads to an infinite
loop where the typo is always inserted in the new token stream without advancing
the token stream.