Fix array access priority in PowerShell lexer

[pyrmont]

The PowerShell lexer expects the text .identifier to be followed by parameters. This is not always the case as in the case where the text is .identifier['key']. This creates a new rule for handling this case.

This fixes #1407.

[pyrmont]

@flipcc I initially thought the problem was in the :dq state but after going through the example you provided more closely, I realised that the issue was the lexer getting stuck in the :parameters state. This was happening because the lexer was treating .Properties as being a function that it expected to be followed by parameters.

I'm not terribly happy with my solution. It feels a bit like a band-aid but what do you think? In your example, what is Properties? Is it a function? Is Properties['sn'] an example of array access?

[flipcc]

To be honest, I am having difficulties wrapping my head around the implementation of lexers, so I am not qualified to judge if it is a bandaid kind of solution. But your comment made it a good bit clearer what is happing and I am confident that we are looking at the right spot now.
You are right in your assumption that .Properties is not me chaining/calling a function.