Fix support for flow optional parameters in arrow functions

[danez]

This fixes cases like this:

var f = (x?) => {}

The main issue was that parseMaybeConditional was detecting the question mark as start of a ternary operator, and failed because it obviously is not one.

To fix it parts of parseMaybeConditional was split into parseConditional and then overwritten in the flow plugin. There the main implementation is called, but syntax errors are catched and the state reseted to the question mark. The question mark can then be handled in parseParenItem.
The rest of the changes is to ensure that the optional operator only works in arrow functions.

Fixes T7096