Extend dynamically generated selectors (summary).

This ticket is to replace (mostly for the sake of clear minimal examples): #1325 #1539 #1597

It includes two distinct sub-features that require internal selector re-parsing:

---

[1]. Extend interpolated selector:

@var: a;

@{var} {
  color: red;
}

b:extend(a) {}

Expected result:

a, b {
  color: red;
}

---

[2]. Extend concatenated selector:

a {
  &b {
    color: blue;
  }
}

c:extend(ab) {}

Expected result:

ab, c {
  color: blue;
}

---

Related but pretty much independent feature of using a variable as an extend argument, i.e. :extend(@{var}), is clearly specified in #1485 and not included here.

[zxcabs]

🆙

[pcwinters]

I can imagine that this requires fundamental changes to some internals, but it should really be higher priority. 👍