Add support for `NLPEndpoint`

[jdkato]

I'd like to automate the process outlined in vale-cli/vale#356: essentially, have the extension start/manage the spaCy server so that users don't have to do it manually.

I'd also like to include some default models for a few languages.

[ChrisChinchilla]

@jdkato Personally I'd like something of an explainer on the new NLP features, as they are obviously extremely useful but I am quite behind on keeping up with the changes you've made.

[jdkato]

In previous releases Vale's NLP-related features only supported my own, English-only library.

Now, you can use spaCy, which has support for many different languages with great speed + accuracy -- basically Vale is now multilingual.

[jdkato]

An update: I'm still working out the details of packaging the Python API. It's looking like I'm going to go with PyInstaller -- I've added a sample release (https://github.com/errata-ai/spacy-vale/releases) with the API packaged into a standalone folder (macos.zip).