Added |truncate filter

[yacir]

This adds the missing |truncate filter.

[mitsuhiko]

I'm in favor of adding it, but I would prefer that filter to land in minijinja-contrib given the rather complex implementation and that it's probably not used very frequently.

[yacir]

I understand your reasoning. Since this filter is native to Jinja2, it makes sense to consider adding it to the builtins. However, given the potential infrequent use, it might be more practical to include it in minijinja-contrib. I'll update the merge request.
Perhaps this could be revisited in v3.

[yacir]

replaced by #647