Add `rownumber`

[VEZY]

Add rownumber from DataFrames. See #60

[codecov[bot]]

Codecov Report

Patch and project coverage have no change.

Comparison is base (7fa794b) 96.36% compared to head (6596edd) 96.36%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #61 +/- ## ======================================= Coverage 96.36% 96.36% ======================================= Files 1 1 Lines 55 55 ======================================= Hits 53 53 Misses 2 2 ``` | [Impacted Files](https://app.codecov.io/gh/JuliaData/DataAPI.jl/pull/61?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaData) | Coverage Δ | | |---|---|---| | [src/DataAPI.jl](https://app.codecov.io/gh/JuliaData/DataAPI.jl/pull/61?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=JuliaData#diff-c3JjL0RhdGFBUEkuamw=) | `96.36% <ø> (ø)` | |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[bkamins]

@quinnj, @nalimilan - OK to merge?

[bkamins]

Thank you!