skip parsing and formatting custom CSS properties

[realStandal]

Untested

Closes #423

Skips formatting custom CSS properties using a lax check that a property's key does not begin with --.

[vercel]

This pull request is being automatically deployed with Vercel (learn more).
To see the status of your deployment, click below or on the icon next to each commit.

🔍 Inspect: https://vercel.com/cristianbote/goober-rocks/412qkvrbwnKbfQ3DZ1mQoGFHTSJX
✅ Preview: https://goober-rocks-git-fork-realstandal-master-cristianbote.vercel.app

[codesandbox-ci]

This pull request is automatically built and testable in CodeSandbox.

To see build info of the built libraries, click here or the icon next to each commit SHA.

Latest deployment of this branch, based on commit 7a2217f:

Sandbox	Source
Vanilla	Configuration

[realStandal]

I've redone the checking to exclude replacing CSS variables so old tests pass, and have included some new ones.

I'm not sure of the consequences of parsing the variables, but it hasn't caused me a problem with simple values 🤷‍♀️

[ksenginew]

Shaved some bytes for you.

[cristianbote]

Thank you @realStandal and @ksenginew!