[RFC]: Add implementation for `@stdlib/stats/base/dists/erlang/logcdf`

### Description

This RFC proposes to add the implementation for calculating the logarithm of cumulative distribution function ( CDF ) for Erlang distribution.

### Related Issues

Related issues # , # , and # .

### Questions

No.

### Other

No.

### Checklist

- [x] I have read and understood the [Code of Conduct](https://github.com/stdlib-js/stdlib/blob/develop/CODE_OF_CONDUCT.md).
- [x] Searched for existing issues and pull requests.
- [x] The issue name begins with `RFC:`.

Start by locating existing distribution implementations in the stdlib statistics package and compare how logarithmic CDF functions and their tests are organized. Implement the Erlang distribution's log-CDF behavior and add matching coverage; done means the implementation and tests follow the repository's established distribution patterns.

索引モデルが issue の本文から書いたものです。