[Fix #4719] Detect tabs between string literals

[buehmann]

This fixes #4719 and simplifies Layout/Tab code.

The problem was that entire :dstr expressions were considered "inside a string", whereas they can be composed of multiple individual string literals:

"foo" \
	"bar"

s(:dstr,
  s(:str, "foo"),
  s(:str, "bar"))
"foo" \
~~~~~~~... expression

s(:str, "foo")
"foo" \
    ~ end
~ begin
~~~~~ expression

s(:str, "bar")
        "bar"
            ~ end
        ~ begin
        ~~~~~ expression

In the AST this looks very similar to a multiline string, but the begin (and end) locations differ:

"foo
	bar"

s(:dstr,
  s(:str, "foo\n"),
  s(:str, "\tbar"))
"foo
~ begin
~~~~... expression
        bar"
           ~ end

s(:str, "foo\n")
"foo
 ~~~... expression

s(:str, "\tbar")
        bar"
       ~~~~ expression

[koic]

Thanks!

[bquorning]

@buehmann I would be interested in hearing your opinion on the somewhat related issue #6420 too.