Add primitive array support for filter function.

**Is your feature request related to a problem? Please describe.**

The filter function does not support filtering primitive arrays like below
```filter(["foo", "bar", "foobar"], length(it)==3) ```

This is mainly because it does not support the `it` reference

**Describe the solution you'd like**

Add support for `it` operator in filter function, so that it supports primitive array

Start by locating the filter function implementation and the handling of the `it` reference in the expression evaluator. Reproduce the primitive-array example from the issue, then add coverage showing that `filter(["foo", "bar", "foobar"], length(it)==3)` works and verify existing filter behavior remains unchanged.

Written by the indexing model from the issue text.