Add sway support to Mason

From @pedronauck on Slack:

Hey folks, any change of getting Sway on Mason? [https://github.com/williamboman/mason.nvim/](https://github.com/williamboman/mason.nvim/)
It will be so easier to install Sway on Neovim and get it running in the properly way, now it's not so easy to accomplish this as is for another LSPs.

The request mentions Mason and Neovim but names no files or tests in sway.vim. Start by checking Mason's support requirements and how Sway is currently installed; done means Sway can be installed through Mason and runs correctly in Neovim.

Written by the indexing model from the issue text.