feat: :Elixir command with subcommands and completions

Closes #141

Doesn't fully fix it, but starts it and I don't think I'll migrate the
ElixirLS ones to it.

Author: mhanberg

README.md

@@ -147,21 +147,45 @@ elixir.setup {
 
 # Features
 
+## Commands
+
+:Elixir {command} [{subcommand}]
+
+: The main elixir-tools command
+
+      ```vim
+      :Elixir nextls uninstall
+      ```
+### Full List
+
+| Command | Subcommand | Description                                                                                          |
+|---------|------------|------------------------------------------------------------------------------------------------------|
+| nextls  | uninstall  | Removes the `nextls` executable from the default location: `~/.cache/elixir-tools/nextls/bin/nextls` |
+
 ## Next LS
 
 > **Note**
 > Next LS is **disabled** by default. Once it reaches feature parity with ElixirLS, it will switch to enabled by default.
 
 > **Note**
-> Next LS integration utilizes `Mix.install/2`, so you must be running Elixir >= 1.12
-
-> **Note**
-> Next LS creates a `.elixir-tools` directory in your project root. You'll want to add that to your gitignore.
+> Next LS creates a `.elixir-tools` directory in your project root, but it's automatically ignored by git.
 
 The language server for Elixir that just works. 😎
 
 You can read more about it at https://www.elixir-tools.dev/next-ls.
 
+### Automatic Installation
+
+Next LS is distributed as pre-compiled binaries, which are available from the Next LS GitHub releases page. elixir-tools.nvim will prompt you to install it if it is not found, and then
+will consequently download it from GitHub.
+
+If you are using a package manager like [Mason](https://github.com/williamboman/mason.nvim), you can set the `cmd` property of the `nextls` setup table
+and it will not prompt you to install and use it from there.
+
+### Commands
+
+Next LS command are available as subcommands of the `:Elixir` command
+
 ## Credo Language Server
 
 > **Note**

lua/elixir/init.lua

@@ -1,3 +1,7 @@
+if not vim.iter then
+  vim.iter = require("elixir.iter")
+end
+
 local elixirls = require("elixir.elixirls")
 local credo = require("elixir.credo")
 local nextls = require("elixir.nextls")
@@ -22,13 +26,52 @@ local enabled = function(value)
   return value == nil or value == true
 end
 
+local define_user_command = function()
+  vim.api.nvim_create_user_command("Elixir", function(opts)
+    local args = vim.iter(opts.fargs)
+    local command = args:next()
+    local not_found = false
+
+    if "nextls" == command then
+      local subcommand = args:next()
+      if "uninstall" == subcommand then
+        vim.fn.delete(nextls.default_bin)
+        vim.notify(
+          string.format("Uninstalled Next LS from %s", nextls.default_bin),
+          vim.lsp.log_levels.INFO
+        )
+      else
+        not_found = true
+      end
+    else
+      not_found = true
+    end
+    if not_found then
+      vim.notify("elixir-tools: unknown command: " .. opts.name .. " " .. opts.args, vim.lsp.log_levels.WARN)
+    end
+  end, {
+    desc = "elixir-tools main command",
+    nargs = "+",
+    complete = function(_, cmd_line)
+      local cmd = vim.trim(cmd_line)
+      if vim.startswith(cmd, "Elixir nextls") then
+        return { "uninstall" }
+      elseif vim.startswith(cmd, "Elixir") then
+        return { "nextls" }
+      end
+    end,
+  })
+end
+
 function M.setup(opts)
   opts = opts or {}
 
   opts.elixirls = opts.elixirls or {}
   opts.credo = opts.credo or {}
   opts.nextls = opts.nextls or {}
 
+  define_user_command()
+
   if not opts.credo.cmd then
     opts.credo.cmd = M.credo.default_bin
   end

lua/elixir/nextls/init.lua

@@ -5,10 +5,6 @@ if not vim.uv then
   vim.uv = vim.loop
 end
 
-if not vim.iter then
-  vim.iter = require("elixir.iter")
-end
-
 M.default_bin = vim.env.HOME .. "/.cache/elixir-tools/nextls/bin/nextls"
 
 function M.setup(opts)