Expose internal api.

- add `exposed_state` to expose 'current_node' and 'cursor_pos'
for a current buffer to the user.
- add `get_buf_state` and `get_node_api` for users.
- add documentation about api functions.
- remove `node_movement` module which should be done in user side.

lua/nvim-treesitter.lua

@@ -5,6 +5,7 @@ local utils = require'nvim-treesitter.utils'
 local info = require'nvim-treesitter.info'
 local configs = require'nvim-treesitter.configs'
 local state = require'nvim-treesitter.state'
+local ts_utils = require'nvim-treesitter.ts_utils'
 
 local M = {}
 
@@ -55,4 +56,13 @@ function M.statusline(indicator_size)
   end
 end
 
+function M.get_buf_state()
+  local bufnr = api.nvim_get_current_buf()
+  return state.exposed_state(bufnr)
+end
+
+function M.get_node_api()
+  return ts_utils
+end
+
 return M