Function uniqueId always returns a 12 chars id

[alxscms]

The function uniqueId was making this library "impure". The output of the minifier could be different from one call to another using the same input and using the option maxLineLength.

Let me explain:

The function joinResultSegments is relying on the segments length to decide whether to add a newline separator or not when joining them while using the option maxLineLength. As the function uniqueId was generating a random length ID, sometimes you could have different outputs with HTML elements being moved to a new line while it wasn't on a previous call.

I changed the code of uniqueId a bit so that the returned ID is always a 12 chars string.

[alexlamsl]

Please provide a test case (which may not fail all the time due to the random nature of length returned by uniqueId()) before we proceed.

For instance, minify('<div><?foo?></div>', { maxLineLength: 5 }) does not yield any difference with or without this change over a million runs.