Ensure inserting at position 0 is consistent with non-zero positions.

[eventualbuddha]

When inserting at non-zero positions the string is placed after any previous strings that would have been placed at that position. Before this commit, inserting at position 0 was treated specially, deferring to the prepend operation. This commit changes the behavior of inserts at 0 to match inserts at any other position. If the old behavior is desired, prepend should be called directly.

For context, this is an issue for decaffeinate because, given this CoffeeScript:

a = 1
b = c = 2

I would like the final output to be this:

var c;
var a = 1;
var b = c = 2;

However, what's actually being output is this:

var var c;
a = 1;
var b = c = 2;

decaffeinate inserts var c;\n at 0 first because it wants that to be above any other statements. It then later inserts var also at 0 (right before a), expecting it to appear after the previously-inserted variable declaration. That isn't happening, but with this patch it does.

[Rich-Harris]

Thanks - released in 0.4.7