Add spacing presets to theme.json

[madhusudhand]

Ref: #334, #366

This PR adds spacing presets to theme.json which creates preset css variables.
SASS function spacing() is replaced with the --wp--preset--spacing--* variables, and all css files are replaced by preset variables.

Steps to test:

1. In the editor, verify the new spacing presets are available in the spacing tools.

2. Verify the theme and all available blocks. They should not get effected.

Considerations:

• It uses same schematics quarter, half, single, double and quad
• However it is recommended to use relative naming such as x-small, small, medium etc..

[jaymcp]

This looks great to me. Thanks for the PR!