[Feature Request]: Add `keep_character` parameter to `is_categorical` function

### Feature description

Currently the function keeps both factors and characters objects when filtering, however, in `tmg` there are situations where we are only interested in `factors` in order to exclude fields such as `STUDYID` or `USUBJID`.

If we want to exclude them currently we need to set `min.length = 2` (exclude singular values) and `max.length = 10` (exclude above a certain threshold)

However, it also excludes fields such as age unit (`AGEU`).

### Suggestion

```r
teal.picks::is_categorical(min.len, max.len, keep_factors = TRUE, keep_characters = TRUE)
```

### Code of Conduct

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### Contribution Guidelines

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### Security Policy

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Start by locating the `is_categorical` function and inspect how it currently filters factor and character objects using `min.len` and `max.len`. Add the requested option so factors can be retained independently of characters, then verify that fields such as `STUDYID`, `USUBJID`, and `AGEU` produce the intended filtering behavior.

Written by the indexing model from the issue text.