''
'fo\'obar'
'foo
$bar
baz'
""
"fo\"obar"
"foo
$bar
baz"

----------------------------------------------------

[
	["string", ["''"]],
	["string", ["'fo\\'obar'"]],
	["string", ["'foo\r\n$bar\r\nbaz'"]],
	["string", [["double-quoted", ["\"\""]]]],
	["string", [["double-quoted", ["\"fo\\\"obar\""]]]],
	["string", [["double-quoted", [
		"\"foo\r\n",
		["interpolation", ["$bar"]],
		"\r\nbaz\""
	]]]]
]

----------------------------------------------------

Checks for strings.
Also checks that string interpolation only applies to double-quoted strings.