Fix SSR issues with SortableJS 1.10.0

[Hugome]

SortableJS 1.10.0 has been release 2 days ago and break some SSR things.
Before 1.10.0 : Every "navigator.useragent"/"typeof window" check was not done on the module import.
Now : When you import sortable it does some client only check :

So the only solution is too import Sortable only on the client side (Like in this PR).
Is this solution really ugly ? I'm not 100 % sure, but the only place this plugin use the Sortable constructor is on the mounted hook, and this hook in SSR is only called on client side (https://ssr.vuejs.org/guide/universal.html) so it will have the module included.

[David-Desmaisons]

Thanks for subimiting this PR.
Did you test this fix to make there is no ther regression in SSR scenario?

[Hugome]

After a little digging i find a problem with this solution, the good old commonjs/es module, import/require black magic.

1st) I forgot the "default" in the require, resolved
2nd) This "default" break tests, resolved by changing the mock to be an esmodule

I try in my application and it seem good
(This fix the new #724 issue)

[David-Desmaisons]

OK, thanks for this. I will take a look today and try to merge in the short term.

[David-Desmaisons]

I made a small chnage to simplify your fix in PR #726

[David-Desmaisons]

Thanks @Hugome

[transtone]

not solve: #732