fix(nvim): annoying indentation for comments in Elixir

When entering a new comment in an indented line, the indentation would
get removed and the comment would get pulled to the beginning of the
line. e.g.:

```elixir
def Foo do
  def hi do
\# hello
    IO.inspect("Hello")
  end
end
```

After the fix
```elixir
def Foo do
  def hi do
    \# hello
    IO.inspect("Hello")
  end
end
```

config/nvim/lua/core/options.lua

@@ -29,7 +29,9 @@ opt.shiftwidth = indent -- Number of spaces to use for each step of (auto)indent
 opt.softtabstop = indent
 opt.expandtab = true --  Use the spaces to insert a <Tab>
 opt.shiftround = true -- Round indent to multiple of 'shiftwidth'
-opt.smartindent = true
+-- this was messing up comments starting with `#`
+-- see https://vim.fandom.com/wiki/Restoring_indent_after_typing_hash
+opt.smartindent = false
 
 -- text appearance
 opt.textwidth = 80 --  set row width size in characters

config/nvim/lua/plugins/treesitter.lua

@@ -72,6 +72,9 @@ M.setup = function()
     },
     autotag = { enable = true },
     highlight = { enable = true },
+    indent = {
+      enable = true
+    },
     context_commentstring = {
       enable = true,
       enable_autocmd = false,