inline negative numbers

[jantimon]

currently negative numbers in styled components were being converted to CSS variables and evaluated at runtime, even though they are constant values

This PR fixes this by inlining negative numbers directly into the CSS output

For example:

const Button = styled.button`
  margin-top: ${-1}px;
`;

Now correctly outputs:

.Button_xyz {
  margin-top: -1px;
}

Instead of using CSS variables and runtime evaluation

• Added test case in constants/input.tsx to verify negative number inlining
• All existing tests pass

Fixes #191