12$: warn on 'x == y;' statement

[strager]

The following C++ code has a bug:

  for (Used_Variable &used_var : current_scope.variables_used) {
    if (used_var.kind == Used_Variable_Kind::use) {
      used_var.kind == Used_Variable_Kind::use_in_type;
    }
  }

JavaScript code might have a similar bug. For example:

let x;
x == 42;

quick-lint-js should warn about such redundant == (or ===) and suggest = instead.

[CoderMuffin]

Hi @strager, I would love to tackle this issue if that's ok :)

[strager]

@CoderMuffin Yeah, you can work on this issue.