Add sex to names

Add genders to existing names. Add new key-value pair `"sex": "string"` to JSON name files.

Use single, lowercase letters: `m` for male, `f` for female and `u` for unisex names. Don't spell out the whole words.

For example:
```
{
"name": "Mary"
"sex": "f",
"meaning": "Most likely Egyptian name derived in part from mry \"beloved\" or mr \"love\". Also known by Greek and Hebrew form of \"sea of bitterness\", \"rebelliousness\", and \"wished for child\"",
...
}
```

You can also:

- [Add a name](https://github.com/bluzi/name-db/issues/1)
- [Translate names to your language](https://github.com/bluzi/name-db/issues/33)
- [Add aliases](https://github.com/bluzi/name-db/issues/87)

Find the JSON name files in the repository and inspect their existing name entries first. Add the requested sex value to the existing names using only m, f, or u, then verify that every updated entry follows the same structure and allowed values.

Written by the indexing model from the issue text.