Fixed Length Pre-Tokenizer

[jonvet]

Introduces a pre-tokenizer to split text in fixed length chunks (closes #1697).
The method pre_tokenize could be more made more concise by creating a vector with indices first like so

    let mut splits = Vec::new();

    for chunk in char_positions.chunks(self.length) {
        let start = chunk.first().map(|(i, _)| *i).unwrap_or(0);
        let end = chunk.last().map(|(i, c)| i + c.len_utf8()).unwrap_or(text.len());
        splits.push(normalized.slice(Range::Normalized(start..end))
            .ok_or("Failed to slice normalized text")?);
    }

but that would take a bit more memory, so I went for my approach instead.