Convert parallel assignment (massign) expressions into multiple assignment expressions

[kddnewton]

Currently, when @prettier/plugin-ruby encounters an massign expression, it will always print it out as an massign. However, there are one case where we don't want to do that:

a, b, c = 1, 2, 3

This is functionally equivalent to:

a = 1
b = 2
c = 3

We should handle this inside the massign handler in src/nodes.js by checking if the number of args on the left is equal to the number of args on the right, that none of them have parens around them, and that none of them are splats. If these conditions are met, the massign expression should be converted into multiple individual assignments.

I'd be happy to pair with someone to accomplish this. It shouldn't take too long once you get the hang of how nodes are being printed.