API offers explicit function calls and implicit when coerced into string

[sndrs]

Sadly, implicitly calling toString won't work in tagged template literals, since coercing the object to strings is not guaranteed - the author of the tag decides what to do.

This adds an explicit function option, while maintaining the overridden toString which can continue to be used outside tagged templates.

It also

• removes most types, since they're tricky and need looking at in a separate pass
• colocates the tests with their modules

[codecov[bot]]

Codecov Report

Merging #20 into master will not change coverage. The diff coverage is 100%.

@@          Coverage Diff          @@
##           master    #20   +/-   ##
=====================================
  Coverage     100%   100%           
=====================================
  Files           3      3           
  Lines          32     45   +13     
  Branches        1      1           
=====================================
+ Hits           32     45   +13

Impacted Files	Coverage Δ
src/index.ts	100% <100%> (ø)	:arrow_up:
src/media-queries.ts	100% <100%> (ø)	:arrow_up:
src/config.ts	100% <100%> (ø)	:arrow_up: