Change [Parameter] warning (BL0004) to be an error - Non-public parameters

[NTaylorMullen]

In preview8 we added a warning for non-public [Parameter]s. In preview9 we need to make this an error.

This is tracking work initially mentioned here.

[Joe4evr]

Just to note, there are apparently people who want to have internal [Parameter]s: #12271.