feat(typescript): Improved theme typing and tests

[luke-john]

BREAKING CHANGE: The updated theme typing will complain if existing usage of theme is unsafe

What:
Updated the typings around themes to get better safety around usage

Why:
We're implementing theming in a component library and wanted safety.

How:
Updated the typescript definition files and tests.

Checklist:

• Documentation
• Tests
• Code complete
• Added myself to contributors table N/A
• Followed the commit message format

[codecov-io]

Codecov Report

Merging #160 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff          @@
##           master   #160   +/-   ##
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  Coverage     100%   100%           
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  Files          10     10           
  Lines         138    138           
  Branches       35     35           
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  Hits          138    138

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[ErikCupal]

Great! 😃

Since Typescript 2.3 it's possible to set default generics type microsoft/TypeScript#13487. It's useful here as it cuts off a little boilerplate. See:

// Without the default generics

export interface StyledFunction<Props, Properties> {
  <CustomProps, ThemeProps>(
    // ...
  ): GlamorousComponent<Props & CustomProps>
}

...

const Title = glamorous.h1<{ color: string }, {}>(
  {
    "fontSize": "10px",
    "zIndex": "auto",
  },
  (props) => ({
    "color": props.color,
  }),
);

// With the default generic

export interface StyledFunction<Props, Properties> {
  <CustomProps, ThemeProps = {}>(
    // ...
  ): GlamorousComponent<Props & CustomProps>
}

...

const Title = glamorous.h1<{ color: string }>(
  {
    "fontSize": "10px",
    "zIndex": "auto",
  },
  (props) => ({
    "color": props.color,
  }),
);

I'll commit the changes for this improvement

[kentcdodds]

Thanks friends! So I'd like to avoid a major version bump if I can. At least, if we're doing that we should probably do other breaking changes while we're at it. So I'm going to merge this into next and we can talk about other things we can do.