Add Expects snippets

[pmareke]

Maybe this is interesting to someone else!

https://expects.readthedocs.io

PD: Thanks for this amazing plugin!

[OkelleyDevelopment]

Hi @pmareke, an initial pass on this looks good. However we do need to add new entry to the package.json so that this is available for users to extend under the python filetype.

e.g,

luasnip.filetype_extend("python", {"python-expects"})

Would mean we have in the package.json an entry like:

{
  "language": "python-expects",
  "path": "./snippets/python/expects.json"
}

[pmareke]

Sure, done!