Allow short argument names when using double dashed args

### Background

Currently one-letter arguments are only supported with single-dashed arguments. e.g.

This works:

```powershell
dotnet cake -a=5
```

Whereas this does not work (error: `Invalid long option name`)

```powershell
dotnet cake --a=5
```

### Proposal

This issue intends to make the necessary updates to allow short argument names when using double-dashed arguments.

### Important

Cake uses [Spectre.Console](https://github.com/spectreconsole/spectre.console) for parsing arguments, so there's a little bit of investigation needed to see if the fix can (and should) be done directly in Cake, of if it should be fixed [upstream](https://github.com/spectreconsole/spectre.console). If you discover that the fix doesn't belong in Cake, you'd need to send a PR to Spectre.Console and the fix in Cake would be just upgrading to a newer version of [Spectre.Console](https://github.com/spectreconsole/spectre.console).

---

Related:
- [Feature Request: CLI Help Improvements](https://github.com/cake-build/cake/issues/2331)

Start by reproducing the documented `dotnet cake --a=5` failure and inspect Cake's integration with Spectre.Console argument parsing. Determine whether the change belongs in Cake or upstream; done means the double-dashed one-letter argument is accepted and the appropriate project has the needed update.

Written by the indexing model from the issue text.