update style-loader to 0.21.0

[shanecav]

What kind of change does this PR introduce?

Updates the style-loader package to the latest version (0.21.0).

Did you add tests for your changes?

n/a

Summary

I'd like to be able to target an iframe with the insertInto option of style-loader (using preact.config.js). This allows your styles to be inserted into the iframe's head (see webpack/style-loader#248). The current version of style-loader used by preact-cli (0.18.2) doesn't have this feature - it was introduced in 0.19.0.

The reason I need this is that I'm building an embeddable Preact widget that'll be rendered into the body of a sourceless iframe, and so that's where I'd like my styles to be inserted.

Does this PR introduce a breaking change?

It shouldn't - I couldn't find any breaking changes in the style-loader changelog.

[hassanbazzi]

@shanecav Tests failing. Maybe try merging in latest master?

[hassanbazzi]

We're going to be upgrading this by default in the next branch. Closing since 3.0 is soon. :)