Allow blocks to set their own gap in theme.json

[scruffian]

This is an update to WordPress/gutenberg#37360 which attempts to allow blocks to still set their own values for gap while still using the --wp--style--block-gap variable at the root level.

To test, use a theme that sets a custom blockGap for a block (for example Skatepark has a custom blockGap for the navigation block). Check that:

• The navigation block uses the custom gap specified in theme.json
• The --wp--style--block-gap is still defined
• Try setting a custom gap on a columns block and confirm that the space between blocks isn't affected (Block Spacing setting causes conflict with blockGap (Columns, Navigation, etc.) gutenberg#36521).

[scruffian]

Closing this old PR.