[highlights] Add TSSymbol highlight group

Addresses issue #892

Ruby and Dart literal symbols will now be highlighted by the new
TSSymbol highlight group, which itself will, by default, link to the Vim
Identifier highlight group. Vim theme authors can then set their
TSSymbol colors.

Symbol highlighting can apply to a number of languages as noted in the
following Wikipedia page: https://en.wikipedia.org/wiki/Symbol_(programming)

Not just for Ruby and Dart.

doc/nvim-treesitter.txt

@@ -560,6 +560,10 @@ For regexes.
                                                            *hl-TSStringEscape*
 For escape characters within a string.
 
+`TSSymbol`
+                                                                 *hl-TSSymbol*
+For identifiers referring to symbols or atoms.
+
 `TSTag`
                                                                    *hl-TSTag*
 Tags like html tag names.