parse.y: separate numeric literal

* parse.y (parser_yylex): separate numeric literal from succeeding
  token, and treat 'e' as floating point number only if followed by
  exponent part.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@42197 b2dd03c8-39d4-4d8f-98ff-823fe69b080e