editorconfig: support spelling_language property

[MGreek]

Problem

By default the spelling_language property of an editorconfig file is not supported by neovim.
I noticed this when:

• consulting :h editorconfig inside neovim and seeing that spelling_language isn't listed in the "supported by default" section.
• setting spelling_language to en-US and neovim doesn't set the spell options accordingly

Expected behavior

neovim should react to the spelling_language option that was recently added to the editorconfig spec.