Add lem-extesion-manager

[Sasanidas]

So, with this PR I want to add https://github.com/lem-project/lem-extension-manager for the main Lem repository.

This would be a draft until I add the commands https://github.com/lem-project/lem-extension-manager/blob/main/main.lisp#L225 that are not used on the extension-manager to the main Lem codebase and I would also like to add some documentation.

[Sasanidas]

I have to comment the library, but I think all the basic code is in place, and the package system can start working.

Some improvements would be the ability to update git packages and quicklisp ones (both are quite easy to add).

A lock file may also be nice, tho the idea is to keep the package manager light and easy to use

[Sasanidas]

Doc written https://github.com/lem-project/lem-extension-manager

[Sasanidas]

Ping @cxxxr

[cxxxr]

Sorry, I've been meaning to look at this sometime, but it's been on the back burner for a long time.

[cxxxr]

The word “package” seems to be the unit of extension, but it overlaps with the term “package” in Common Lisp, so I found it difficult to understand.
Wouldn't extension be better?

[cxxxr]

The README says to do it like this, but I'm worried that the information will be duplicated with asd.

                                       :source (:type :git
                                                :url "https://github.com/smithzvk/Versioned-Objects.git"
                                                :branch "advance-versioning")
                                       :dependencies (("ip-management"
                                                       :source
                                                       (:type :git
                                                        :url "https://github.com/Sasanidas/ip-management.git"))
                                                      ("1am" :source (:type :quicklisp))))

[cxxxr]

What is simple-package? The reason it has simple in the name is probably just to avoid a conflict with cl:package. Wouldn't it be better to just call it extension?