feat: string slicing functions

Given that vector indexing is discouraged (it will index the dataframe series, not the string), the following string slicing functions may be useful.

```julia
left(text::AbstractString, n::Int) = n > 0 ? text[1:min(n, end)] : text[1:end-min(-n, end)]
right(text::AbstractString, n::Int) = n > 0 ? text[end-min(n, end)+1:end] : text[min(-n, end)+1:end]
```

also add a `substr`