Fix parsing of "\\\n" escaped sequences in various literals.

[iliabylich]

Closes #537

Introduced Literal#supports_line_continuation_via_slash? method that returns true/false depending on the type of the literal. I wasn't able to find any correlation between interpolation and escaping rules. Looks like that's a completely new separate property of the literal. Ruby ❤️

Added spec that verifies that "\\\n" gets properly escaped for all kinds of literals. @whitequark Please let me know if I should convert it from acceptance table to a list of plain assertions.

test data

$ cat test.rb
def `(str); str; end

p 'a\
b'

p <<'HERE'
  a\
  b
HERE

p %q{a\
b}

p "a\
b"

p <<"HERE"
  a\
  b
HERE

p %{a\
b}

p %Q{a\
b}

p %w{a\
b}

p %W{a\
b}

p %i{a\
b}
p %I{a\
b}

p :'a\
b'

p %s{a\
b}

p :"a\
b"

p /a\
b/

p %r{a\
b}

p %x{a\
b}

p `a\
b`

p <<`HERE`
a\
b
HERE

ruby -e

$ ruby test.rb
"a\\\nb"
"  a\\\n  b\n"
"a\\\nb"
"ab"
"  a  b\n"
"ab"
"ab"
["a\nb"]
["a\nb"]
[:"a\nb"]
[:"a\nb"]
:"a\\\nb"
:"a\\\nb"
:ab
/ab/
/ab/
"ab"
"ab"
"ab\n"

bin/ruby-parse

$ bin/ruby-parse test.rb
warning: parser/current is loading parser/ruby26, which recognizes
warning: 2.6.0-dev-compliant syntax, but you are running 2.6.0.
warning: please see https://github.com/whitequark/parser#compatibility-with-ruby-mri.
(begin
  (def :`
    (args
      (arg :str))
    (lvar :str))
  (send nil :p
    (dstr
      (str "a\\\n")
      (str "b")))
  (send nil :p
    (dstr
      (str "  a\\\n")
      (str "  b\n")))
  (send nil :p
    (dstr
      (str "a\\\n")
      (str "b")))
  (send nil :p
    (str "ab"))
  (send nil :p
    (str "  a  b\n"))
  (send nil :p
    (str "ab"))
  (send nil :p
    (str "ab"))
  (send nil :p
    (array
      (str "a\nb")))
  (send nil :p
    (array
      (str "a\nb")))
  (send nil :p
    (array
      (sym :"a\nb")))
  (send nil :p
    (array
      (sym :"a\nb")))
  (send nil :p
    (dsym
      (str "a\\\n")
      (str "b")))
  (send nil :p
    (dsym
      (str "a\\\n")
      (str "b")))
  (send nil :p
    (sym :ab))
  (send nil :p
    (regexp
      (str "ab")
      (regopt)))
  (send nil :p
    (regexp
      (str "ab")
      (regopt)))
  (send nil :p
    (xstr
      (str "ab")))
  (send nil :p
    (xstr
      (str "ab")))
  (send nil :p
    (xstr
      (str "ab\n"))))