Add simulator

[DutChen18]

I've already seen 2 pull request for "animation viewer" or "preview" features to make it easier to write animations.
Having looked at both in my opinion they're not very good, both are very slow to update and either require running in a web browser or significant modifications to the original code.

This pull request implements a better solution, it:

• is self contained in a seperate file of only 25 lines
• performs better than other implementations
• is a drop in replacement for hardware neopixel, only requiring you to change 2 imports
• does not modify the original code at all

To use it you just need to change 2 lines of code:

    # use real leds
    import board
    import neopixel
    
    # use simulator
    from sim import board
    from sim import neopixel

[Diggsey]

This seems to be the best simulator so far 👍

[oliverdunk]

Hi @DutChen18, thanks for opening this PR! Matt asked me to help with maintaining this repository, and I think having a simulator available would be nice. I love some of the others which all look really polished - but this one stood out for its simplicity.

A few thoughts:

• It would be great if the simulator could be run without changing the code. Would it be possible to add a command line argument? We could then conditionally import the simulator similar to Add yet another simulator #8.
• It would be great if the simulator ran automatically for anyone without neopixel, which I assume will be the vast majority of people.
• Just a little nit but there's a newline missing at the bottom!

Would you be willing to make a few changes based on those? Alternatively, with your permission, I'd be happy to jump in and add a few extra commits. I appreciate that you may not have more time to spend on this.