[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:`.

首先定位 stdlib 统计包中现有的分布实现，并比较对数 CDF 函数及其测试是如何组织的。实现 Erlang 分布的 log-CDF 行为并添加相应的覆盖；当实现和测试遵循仓库中既定的分布模式时，即表示完成。

由索引模型根据 Issue 内容生成。