JavaScript: SPI implementation

[jamisonderek]

What's new

• This adds SPI interface to JavaScript. You can read, write or writeRead data on the SPI bus. writeRead reads data as it writes data, so the output will be the same length as the input. Many chips will ignore any pending commands once Chip Select goes high, so there are acquire and release methods if you want to pull Chip Select low for multiple write/read calls.

This library uses the standard SPI pins (MOSI=pin 2, MISO=pin 3, CS=pin 4, SCK=pin 5) for communication.

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For the reviewer

• I've uploaded the firmware with this patch to a device and verified its functionality
• I've confirmed the bug to be fixed / feature to be stable