Add CORE/CODE configurable styles to BCB

[pbking]

This change adds configurable border (color/style/width/radius) and color (text/background) to the core/code block via custom theme.json configuration.

The code block example for Blank Canvas doesn't seem to be behaving properly (I couldn't say why...) so the styles from the form configuration were used for this block's border configuration value.

Additionally the "padding" attributes applied in _alignment have been refactored slightly to be less specific. As it was a block was unable to define a padding variable without additional specificity hacking. This was discovered while attempting to add custom padding configuration to this core/code block per the linked Gutenberg PR. Additionally that code was (inappropriately) enforcing a top/bottom padding of 0 on all the blocks, even if that wasn't their natural state.

In the Gutenberg change that adds the ability to configure this block additionally the font-stylings were discussed as something to be added. However there wasn't a need to do that and bring the styles in alignment with Blank Canvas so that customization option has been deferred until it's needed (and hopefully that will be after something like this is complete).

Before:

After:

[pbking]

This is no longer needed since it can be done via styles in this branch instead.