treesitter: use new on_bytes interface

This will significantly reduce the parsing work
needed e.g. when rehighlighting after every keypress
in insert mode.

Also add safety check for tree-sitter trying to read
past the end of a line. This can happen after we sent
an incorrect buffer update.

runtime/lua/vim/treesitter/highlighter.lua

@@ -60,7 +60,7 @@ function TSHighlighter.new(query, bufnr, ft)
     ft,
     {
       on_changedtree = function(...) self:on_changedtree(...) end,
-      on_lines = function() self.root = self.parser:parse():root() end
+      on_bytes = function() self.root = self.parser:parse():root() end
     }
   )