Better handle multiline interpolated strings in the parser translator

[Earlopain]

Fixes rubocop/rubocop#12870

For example:

"a
#{}
c
"

The parser gem produces

s(:dstr,
  s(:str, "a\n"),
  s(:begin),
  s(:str, "\n"),
  s(:str, "c\n"))

While the translator wraps the last two string nodes in a dstr. There was already logic that kinda was supposed to handle this but it was too specific, applying only when the interpolation contains a single string node.

Much of this logic should be shared between interpolated symbols and regexps. It's also incorrect when the node contains a literal \\n (same as for plain string nodes at the moment).

After #3373 is merged, the added method for creating string nodes can be used to make it more correct in all of these cases.