Add formatter for `ssort`

[egelja]

A new Python formatter, ssort, has recently be released. The special thing about this one is that is reorganizes source code, not just whitespace/line length/etc. Would it be possible to add this as a formatter, along with black, AutoPEP8, and YAPF?

Note: From the README, ssort has to run before black or isort.

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Thank you very much!

[karthiknadig]

@MrAwesomeRocks We have plans of separating tooling like formatting and linting into their own extension. See #18469 . As a part of this work, we plan on providing an extension template that tool authors can use to build VS Code extensions for their tools. We just posted about a prototype for this on black see here psf/black#2883, and autopep8 here hhatto/autopep8#624. Please do watch for our blogs, we will share information on how to create your own extension for formatting using our extension templates.