Insertion plugin with tree

[tkh44]

What:
Stylis processes styles from the inside out so we need to build some kind of structure to ensure proper insertion order of the rules.

See #384 for more details.

Why:
Nested pseudo selectors were breaking.

How:
Simple tree structure is used. The idea came from the stylis AST plugin.

Checklist:

• Documentation N/A
• Tests
• Code complete

closes #384

[codecov]

Codecov Report

Merging #386 into master will increase coverage by 0.07%.
The diff coverage is 100%.

Impacted Files	Coverage Δ
packages/emotion/src/index.js	100% <100%> (ø)	⬆️

[emmatown]

Is id or parent used here?

[tkh44]

No, they were but not anymore.

[thysultan]

@tkh44 The current AST plugin in the stylis repo could benefit from a lot of optimizations and improvements. For example this gist demos one that preserves nested children within the AST.

Did you find any pain points that could be used to improve the stylis plugin system? I'm thinking of passing a depth argument to have data about what depth a rule in a nested selector is in.