Highlight unicode characters in constants

#138 forgot about constants, and this remedies that mistake. Ruby doesn’t allow unicode characters as the first character of a constant (as shown by the below script), so we only highlight constants which have unicode characters as their second or later letter. I separately validated that all valid unicode characters are allowed (with a trivial modification of this script).

```ruby
unicode_codepoints = 0x80..0x10FFFF

valid_codepoints = unicode_codepoints.select do |codepoint|
  str = [codepoint].pack 'U'
  begin
    eval "class #{str}; end"
  rescue SyntaxError
    false
  else
    true
  end
end

valid_codepoints  #=> []
```