Defer loading `speedbar`

[andyleejordan]

The speedbar package used to be required when powershell.el was evaluated. Instead, we can load it only when powershell-mode is evaluated (and it's an autoload).

The approach was the same as for imenu.

[jschaf]

Thank you for the commit.

[jschaf]

I've also sent you a collaborator invite. Feel free to push directly without a PR. I don't use powershell anymore so I'd like to make it as easy as possible for others to send improvements.

[andyleejordan]

Thanks @jschaf!