Add support for the Lean language

[GaetanLepage]

• add plugins.lsp.servers.lean (leanls)
• add plugins.lean (lean.nvim)

[traxys]

shouldn this be in the global scope and not in infoview ? (as from the README.rst):

  -- Redirect Lean's stderr messages somehwere (to a buffer by default)
  stderr = {
    enable = true,
    -- height of the window
    height = 5,
    -- a callback which will be called with (multi-line) stderr output
    -- e.g., use:
    --   on_lines = function(lines) vim.notify(lines) end
    -- if you want to redirect stderr to `vim.notify`.
    -- The default implementation will redirect to a dedicated stderr
    -- window.
    on_lines = nil,
  },

  -- Legacy Lean 3 support (on_attach is as above, your LSP handler)
  lsp3 = { on_attach = on_attach },

  -- mouse_events = true will simulate mouse events in the Lean 3 infoview, this is buggy at the moment
  -- so you can use the I/i keybindings to manually trigger these
  lean3 = { mouse_events = false },
}

[GaetanLepage]

This is indeed what the documentation suggests, but after looking at the code, this is clearly under infoview.
This isn't the only error I spotted in this documentation section.

[traxys]

Indeed.... I tried to search for a compatibility layer but there does not seem to be any