Handle special characters in property name

[Maxim-Mazurok]

Our formatPropertyName() function doesn't handle ? in property name, which may result in

interface AddChartResponse {
    /** The newly added chart. */
    embedded?Chart?: EmbeddedChart;
}

Originally posted by @Maxim-Mazurok in #208 (comment)

This probably should be handled using regular expressions. Check if a property name contains any non-letter characters and surround it with quotes in that case. Perhaps, quotes also should be handled somehow. And, perhaps, not all non-letter characters should be treated as "quotable".

[HoldYourWaffle]

Purely out of curiosity, what kind of property would have a '?' in its name?

As for checking if a property name should be quoted: the rules for a valid (non-quoted) identifier are well-defined. It should indeed be trivial to check if an identifier is valid without quotes using a regex like this one.
I think we'd be safe to assume that Google will only use ASCII characters, and even if they don't, it won't really hurt to "unnecessarily" quote property names.

[Maxim-Mazurok]

what kind of property would have a '?' in its name?

I think we'd be safe to assume that Google will only use ASCII characters

We can't be 100% sure about that ;)

It should indeed be trivial to check if an identifier is valid without quotes using a regex

Yep, that's a good idea

it won't really hurt to "unnecessarily" quote property names

Actually, it will hurt, dtslint will fail if we do that