feat(variants): add defaults to theme

[Johan-dutoit]

Goal
The idea of this PR is to add support for defaults in the theme object for the specific variant.

Problem
Currently specifying defaults can only be done when creating a variant, which is both limiting (i.e. no defaults for the createText export) and not in the same location as all the other styles (i.e. developers will forget they exist).

Usage

textVariants: {
    defaults: {
      fontFamily: 'Roboto',
      color: 'primaryText',
    },
    title: {
        fontSize: 24
        ...
    },
}

Implementation
Styles will be applied in the following order
{...createVariantDefaults, ...themeVariantDefaults, ...actualVariantStyles},

Tests
Existing tests completed successfully without modifications.
New tests added to ensure the styles are applied (and in the correct order, with overrides).

Closes GH-49

[JoelBesada]

Great work!

Just one note, this will allow you to pass variant="defaults" to a component. I think we should remove "defaults" as a valid option, at least in the prop types.

[Johan-dutoit]

Just one note, this will allow you to pass variant="defaults" to a component. I think we should remove "defaults" as a valid option, at least in the prop types.

I too noticed this, but didn't see much harm.
Let me see if I can squeeze in an Omit somewhere.

[Johan-dutoit]

Added in an omit for defaults (to the types only): /~https://github.com/Shopify/restyle/pull/51/files#diff-d11836bc5f23eb547c1e6e3dd4325c12R80