As we guarantee backwards compatibility throughout a major version, it helps if
we have that version available, so plugins / sketches can check if they are
compatible, and issue a helpful error if they are not. As further convenience,
defining `KALEIDOSCOPE_REQUIRED_API_VERSION` before including `Kaleidoscope.h`
will result in a check being done by Kaleidoscope, and an error printed if it
does not match the API shipped.

Signed-off-by: Gergely Nagy <algernon@keyboard.io>

Use `static_assert` instead of `#error` to report an API mismatch, resulting in
a much more informative error message.

Thanks to @cdisselkoen for the request, and @noseglasses for the `static_assert`
idea!

Signed-off-by: Gergely Nagy <algernon@keyboard.io>