add built-in Image type

[mmoskal]

This is based on code from MakeCode Arcade in particular https://github.com/microsoft/pxt-common-packages/tree/master/libs/screen

This also moved I2C API to the interpreter (no longer runs through Jacdac service)

TODO:

• [x] fonts/text
• [ ] write-image-to-SPI method (or I2C?)
• [x] img`...` literals
• [x] tests