extend non_constant_identifier_names for destructured variable declarations

[pq]

Destructuring doc.

Example:

BAD

var (LAT, LONG) = geoCode('Aarhus');

(double, double) geoCode(String city) => ...;

GOOD

var (lat, long) = geoCode('Aarhus');

(double, double) geoCode(String city) => ...;

[pq]

This should work for pattern-if statements too.

BAD

if (var [int X_COORD, int Y_COORD] = json) return Point(x, y);

GOOD

if (var [int xCoord, int yCoord] = json) return Point(x, y);

[pq]

And pattern-for statements:

BAD

var fns = <Function()>[];
for (var (FIRST, SECOND) = (0, 1); FIRST <= 13; (FIRST, SECOND) = (SECOND, FIRST + SECOND)) {
  fns.add(() {
    print(FIRST);
  });
}

GOOD

var fns = <Function()>[];
for (var (a, b) = (0, 1); a <= 13; (a, b) = (b, a + b)) {
  fns.add(() {
    print(a);
  });
}