Add useComposedCssClasses hook

[cea2aj]

Add the ability to compose CSS classes

This hook allows custom CSS classes to be combined in various ways with the default styling of a given component. Each component will have a composition prop which will determine how the composition behaves. It is a wrapper around css-modules-theme. This PR doesn't integrate the functionality into any components yet since the use of Tailwind is still up in the air, and that would affect the integration. However once that's decided, this utility will be added to all components.

The function seemed to impact performance a bit, but adding useMemo fixes that

J=1639
TEST=manual

Test this functionality in the Searchbar with Tailwind and test that the component's built in styling works as well as custom styling.

[coveralls]

Coverage decreased (-6.1%) to 69.388% when pulling 3f01557 on dev/add-injectable-css into 0612924 on main.

[oshi97]

what is ClassInterface?

[cea2aj]

It's just a generic. I can switch to T since I think that would make it clearer

[oshi97]

Should the generic extend some interface or does it really let you pass anything?

[oshi97]

oh I'm also cool calling it ClassInterface instead of just T which imo is not as descriptive! My brain just derped out and didn't recognize it as a generic

[cea2aj]

The ClassInterface should be an object with only strings as keys and strings as values (essentially a Record<string,string>). However using that Record causes an issue for Typescript because interfaces with a specific set of keys cannot be indexed by arbitrary strings. Each class interface will have an interface with a specific set of keys, and that's not compatible with Record<string,string>. I created the isThemeObject method so that I could confirm that the builtInClasses and the customClasses objects can be safely indexed by any string. The Theme object from css-modules-theme is simply interface Theme { [key: string]: string; }.