Fix simplification of calc() expressions

[echocrow]

Dart Sass 1.40.0 introduced a new feature that simplifies calc() expressions. Unfortunately, at least part of that simplification logic seems to yield incorrect results.

Example input:

.example {
  opacity: calc(1 - (1 - var(--my-var, 0)));
}

Expected output:

.example {
  opacity: calc(1 - 1 + var(--my-var));
}
// or ideally:
.example {
  opacity: var(--my-var);
}

Actual output (as of v1.40.0 – v1.40.1):

.example {
  opacity: calc(1 - 1 - var(--my-var));
}

Seems like there is missing logic around resolving brackets () in the subtrahend (right-hand side of a substraction); here, + and - may need to be flipped.
For example, a - (b - c) can be simplified to a - b + c, but must not be simplified to a - b - c.

[nex3]

Thanks for the report!