add mypy extension

[Zenor27]

Add mypy typechecker extension for Zed.

• Uses pygls as a simple LSP (since mypy doesn't have one)
• Highlight errors in Zed
• Does not support dmypy (yet)

This is a first version of the extension, and my first extension for Zed.

https://github.com/Zenor27/zed-mypy

[cla-bot]

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