feat: provide a statusline indicator

It will show the current branch at the cursor going the tree as such.
root->node->subnode->leaf

If an argument is provided to `statusline`, then the tree will be
truncated as follows :
..->subnode->subnode

autoload/nvim_treesitter.vim

@@ -0,0 +1,3 @@
+function! nvim_treesitter#statusline(len)
+  return luaeval("require'nvim-treesitter'.statusline(_A)", a:len)
+endfunction

lua/nvim-treesitter.lua

@@ -24,4 +24,35 @@ function M.setup(lang)
   end
 end
 
+function M.statusline(indicator_size)
+  local indicator_size = indicator_size or 1000
+  local expr = require"nvim-treesitter.utils".expression_at_point()
+  local current_node =
+    require'nvim-treesitter.node_movement'.current_node[api.nvim_get_current_buf()]
+
+  local indicator = ""
+  while expr and (#indicator + #(expr:type()) + 5) < indicator_size do
+
+    local prefix = ""
+    if expr:parent() then
+      prefix = "->"
+    end
+
+
+    if expr == current_node then
+      indicator = string.format("%s[%s]%s", prefix, expr:type(), indicator)
+    else
+      indicator = prefix .. expr:type() .. indicator
+    end
+
+    expr = expr:parent()
+  end
+
+  if expr then
+    return "..." .. indicator
+  else
+    return indicator
+  end
+end
+
 return M

lua/nvim-treesitter/parsers.lua

@@ -5,11 +5,12 @@ local M = {}
 
 function M.has_parser(lang)
   local lang = lang or api.nvim_buf_get_option(0, 'filetype')
+  if not lang or #lang == 0 then return false end
   return #api.nvim_get_runtime_file('parser/' .. lang .. '.*', false) > 0
 end
 
 function M.get_parser(bufnr, lang)
-  if M.has_parser() then
+  if M.has_parser(lang) then
     local buf = bufnr or api.nvim_get_current_buf()
     local lang = lang or api.nvim_buf_get_option(buf, 'ft')
     if not M[buf] then