Detect unnecessary $ in enclosed variables

[rodjek]

${$foo} is valid code, but the second $ is unnecessary and should be detected and fixed. This should also be added to the style guide.

/cc #674 #655

[2fa]

What about variables manipulations inside the string?

Simple example:

$foo = 25
$bar = "${foo}" # 25
$baz = "${$foo}" # 25
$qux = "${$foo + 200}" # 225
$quux = "${foo + 200}" # fail