Support non-null assertion operator after array dereference

[mjambon]

This code was found in the wild:

lastDstToken.parts[0]! += token.parts[0];

The parser is confused by the exclamation mark. A simpler test case that also fails is:

a[0]! += 2

and also

a[0]! = 2

This feature would be useful if the typescript type checker could show a warning when a[0] could be unset, causing an error when adding something to it. I haven't managed to obtain such warning in the typescript playground, so I wonder if this syntax is widely used.