Extend Slime Export Commands to export more defclass and defstruct symbols

[VisenDev]

Add slime commands like slime-export-symbol-at-point for easily exporting symbols from one's lisp package. This adds the chosen symbols to the package's :export clause.

[vindarel]

It exists as lisp-add-export.

https://lem-project.github.io/usage/common_lisp/#export-symbols (just added)

but unlike Slime's, it doesn't export all the accessors (or readers/writers) of a defclass (or a structure). That would be a nice addition.

See also Alt-x lisp-organize-imports

[VisenDev]

I see, well the C-c x keybinding should be added then as well