Lint rule: required-subscript-indentation

[wcjohnson]

Add a lint rule that enforces LSC 0.5.9-style subscript indentation:

// illegal
a
.b
// legal
a
  .b

[rattrayalex]

I'd be strongly in favor of this over #94

It'd be very confusing and frustrating to be writing this:

foo
.bar
.baz

and find this to do something completely different without throwing an error:

foo
.bar
['baz']

we should avoid that situation very strongly.

I'm also in favor of generally strong language style standards; I don't know of a reason not to recommend the indented style, which is more standard in the JS community (eg; prettier does this and I'm not aware of major requests to opt out of it).