Feature request: `:Rest` command

[NTBBloodbath]

I think a :Rest command will be great for rest.nvim to evolve and keep growing in the future. This will replace current <Plug>RestNvim commands family, we are going to raise a deprecation warning using vim.deprecate() function (requires #133) at first then delete them after a while.

Improvements provided by :Rest command over current commands:

• Being able to use it without having to create a keybind first
• Completion of the command and its possible arguments
• Greater extensibility and scaling
• Being able to create such command only for http buffers

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Therefore, this will become a breaking change in the future. However, :Rest command will keep the same arguments as current <Plug>RestNvim commands family, that means for running last rest request you could :Rest last or :Rest preview to preview a request without executing it.