Snackbar: add z-index variable

[anderssonjohan]

New feature motivation

When customizing the z-index of components such as the limel-dialog, showing a snackbar can place it beneath the dialog. To make the feature set consistent, the snackbar component should also have a z-index variable.

New feature description

• [ ] allow customization of the MDC snackbar z-index property using a CSS variable
• [ ] update the docs with a value greater than the one of the dialog component

New feature implementation

The index.html for the docs should probably be updated to contain the added --snackbar-z-index variable:

:root {
    --dialog-z-index: 110;
    --popover-z-index: 115;
    --dropdown-z-index: 120;
    --snackbar-z-index: 125;
}

Use the Snackbar z-index sass mixin to set it.