Add command aliases to `README.md` output by `wp scaffold package-readme`

[baizmandesign]

The README.md output by wp scaffold package-readme does not include @alias directive information (alternative names for a given command as described here). I think it would be good to include it either by default (my preference) or via a flag such as below:

wp scaffold package-readme --include-aliases

[swissspidy]

Makes sense to me!

We just recently enabled this for the handbook already via wp-cli/wp-cli#6147 and wp-cli/handbook#605 so this should be trivial to do.

We can include this by default, with a simple line like:

**Alias:** `xyz`

[baizmandesign]

@swissspidy Thanks! Out of curiosity, are multiple command aliases allowed? It appears only the first mention of @alias in the command definition is honored.

[swissspidy]

Only a single alias is supported at the moment.

[baizmandesign]

Thanks @swissspidy!