EOF characters should be allowed as heredoc delimiter

[pocke]

Problem

^D, ^Z, and \0 should be allowed as heredoc delimiter if it is surrounded with quotes, but isn't.

For example:

# test.rb
puts <<~RUBY
  <<~'\0'
  \0
RUBY

$ ruby test.rb | ruby -c
Syntax OK

$ ruby test.rb | ruby-parse /dev/stdin 
/dev/stdin:1:1: fatal: unterminated string meets end of file
/dev/stdin:1: <<~'

[pocke]

By the way, I found other bugs during grep c_any in lexer.rl.
Both of the following are accepted by CRuby, but ruby-parse raises an error unexpectedly.

• /^D/
• ?^D

Note that ^D means 0x04.

Maybe there are other bugs related to c_any char class in lexer.rl but didn't investigate it deeply.

ref: #808