refacto/feat: better handling of parser updates

features:
- node_movement is moving between scopes.
- add selection initialization from normal mode
- add a decremental selection

improvements:
- attach to buffer to run tree parsing on change
- run state update on CursorMoved
- the buffer state is:
```
{
  cursor_pos = { row=row, col=col },
  current_node = node_under_cursor,
  selection = {
      range = nil, -- activates when starting a selection
      nodes = {} -- filling up when starting an incremental selection
  },
  parser = parser, -- parser for current buffer
}
```
- refacto all the modules reliant on parsing the tree, update the current nodes, get the current nodes...

fixes:
- fix has_parser to look for .so libraries
- fix should select the whole file when selection root in selection

lua/nvim-treesitter/state.lua

@@ -0,0 +1,114 @@
+local api = vim.api
+
+local utils = require'nvim-treesitter.utils'
+
+local M = {}
+
+local buffers = {}
+
+local g_mode = api.nvim_get_mode().mode
+
+local function get_selection_range()
+  local _, vstart_row, vstart_col, _ = unpack(vim.fn.getpos("v"))
+  local _, cursor_row, cursor_col, _ = unpack(vim.fn.getpos("."))
+  if vstart_row < cursor_row then
+    return vstart_row, vstart_col, cursor_row, cursor_col
+  else
+    return cursor_row, cursor_col, vstart_row, vstart_col
+  end
+end
+
+function M.update()
+  local bufnr = api.nvim_get_current_buf()
+  local buf_config = buffers[bufnr]
+  if not buf_config then return end
+
+  local mode = api.nvim_get_mode().mode
+  local cursor = api.nvim_win_get_cursor(0)
+  local row = cursor[1]
+  local col = cursor[2]
+  if row == buf_config.cursor_pos.row
+    and col == buf_config.cursor_pos.col 
+    and mode == g_mode 
+    then
+    return
+  end
+
+  local root = buf_config.parser.tree:root()
+  if not root then return end
+
+  local new_node = root:named_descendant_for_range(row - 1, col, row - 1, col)
+
+  if new_node ~= buf_config.current_node then
+    buf_config.current_node = new_node
+  end
+
+  -- We only want to update the range when the incremental selection has not started yet
+  if mode == "v" and #buf_config.selection.nodes == 0 then
+    local row_start, col_start, row_end, col_end = get_selection_range()
+    buf_config.selection.range = { row_start, col_start, row_end, col_end }
+  elseif mode ~= "v" then
+    buf_config.selection.nodes = {}
+    buf_config.selection.range = nil
+  end
+
+  g_mode = mode
+  buf_config.cursor_pos.row = row
+  buf_config.cursor_pos.col = col
+end
+
+function M.insert_selection_node(bufnr, range)
+  local buf_config = buffers[bufnr]
+  if not buf_config then return end
+
+  table.insert(buffers[bufnr].selection.nodes, range)
+end
+
+function M.pop_selection_node(bufnr)
+  local buf_config = buffers[bufnr]
+  if not buf_config then return end
+
+  table.remove(
+    buffers[bufnr].selection.nodes,
+    #buffers[bufnr].selection.nodes
+    )
+end
+
+function M.run_update()
+  local cmd = "lua require'nvim-treesitter.state'.update()"
+  api.nvim_command('autocmd NvimTreesitter CursorMoved * '..cmd)
+end
+
+function M.attach_to_buffer(ft)
+  local bufnr = api.nvim_get_current_buf()
+  local ft = ft or api.nvim_buf_get_option(bufnr, 'ft')
+
+  if buffers[bufnr] then return end
+
+  local parser = utils.get_parser(bufnr, ft)
+  if not parser then return end
+
+  buffers[bufnr] = {
+    cursor_pos = {},
+    current_node = nil,
+    selection = {
+      range = nil,
+      nodes = {}
+    },
+    parser = parser,
+  }
+
+  M.update()
+  api.nvim_buf_attach(bufnr, false, {
+      -- TODO(kyazdani): on lines should only parse the changed content
+      -- TODO(kyazdani): add a timer to avoid too frequent updates
+      on_lines = function(_, buf) buffers[buf].parser:parse() end,
+      on_detach = function(bufnr) buffers[bufnr] = nil end,
+    })
+end
+
+function M.get_buf_state(bufnr)
+  return buffers[bufnr]
+end
+
+return M