Add abstract as a reserved keyword

To support class highlighting like this one:

```ruby
# Base class for nodes in the grammar.
abstract class ASTNode
  property location

  def clone
    clone = clone_without_location
    clone.location = location
    clone
  end

  def name_column_number
    @location ? @location.column_number : nil
  end

  def name_length
    nil
  end

  def nop?
    false
  end

  def to_s_for_macro
    to_s
  end
end
```