Fix string.format "q" option handling of infinity and NaN

[Kan18]

The q specifier should format numbers such that they are a valid constant in Lua code. Previously, Infinity and NaN fell back to Double.toHexString which returns "Infinity" or "NaN", so these cases should be handled separately (matching PUC Lua behavior here).

[SquidDev]

Mostly noting this down because I was curious what the right behaviour was here:

• Lua 5.1/5.2 implement %q by converting the value to a string and quoting it. This means ("%q"):format(0/0) evaluates to "nan" (or possibly "-nan").
• Lua 5.3+ (whose behaviour we attempt, and fail, to mimic), tries to convert strings, numbers and literals to their literal form.
• Lua 5.4 extends that slightly, with special handling for quoting floats (the 1e999 behaviour).

I kinda wonder if we should do this properly and add support for floating point hex literals.

[SquidDev]

I've added support for hex literals, but occurs to me doing %q properly also requires supporting %a/%A (or at least a subset of it). This doesn't actually look too hard to get right, which is a relief at least.

[SquidDev]

Sorry for the delay in looking at this, looks good to me! Thank you <3.