Consistently export `stylesheet`

[dougalg]

Add an export to expose the CSS stylesheet to the end user with the stylesheet name for all use cases.

This reduces code duplication, for cases where the CSS was previously exported twice, and also makes the interface simpler and more consistent for users with a variety of use cases who may need access to the stylesheet directly.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Comparison is base (118253e) 92.94% compared to head (d96e20d) 93.31%. Report is 2 commits behind head on master.

:exclamation: Current head d96e20d differs from pull request most recent head ab777c7. Consider uploading reports for the commit ab777c7 to get more accurate results

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #456 +/- ## ========================================== + Coverage 92.94% 93.31% +0.36% ========================================== Files 10 10 Lines 326 329 +3 Branches 115 116 +1 ========================================== + Hits 303 307 +4 + Misses 22 21 -1 Partials 1 1 ```

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[dougalg]

@egoist Is this something you can help take a look at?

[socket-security[bot]]

Updated and removed dependencies detected. Learn more about Socket for GitHub ↗︎

Packages	Version	New capabilities	Transitives	Size	Publisher
resolve	1.20.0...1.22.8	None	+0/-4	145 kB	ljharb
node-sass	5.0.0...9.0.0	environment	+5/-182	4.04 MB	xzyfer
autoprefixer	10.2.5...10.4.16	shell	+2/-11	275 kB	ai
xo	0.35.0...0.56.0	environment	+13/-597	44.3 MB	sindresorhus

🚮 Removed packages: @babel/core@7.13.8, @babel/preset-env@7.13.9, babel-core@7.0.0-bridge.0, babel-jest@26.6.3, chalk@4.1.0, concat-with-sourcemaps@1.1.0, cssnano@5.0.1, eslint-config-rem@4.0.0, fs-extra@9.1.0, import-cwd@3.0.0, jest@26.6.3, p-queue@6.6.2, pify@5.0.0, postcss@8.4.32, postcss-load-config@3.0.1, postcss-modules@4.0.0, promise.series@0.2.0, rollup@2.40.0, rollup-plugin-postcss@3.0.0, rollup-pluginutils@2.8.2, safe-identifier@0.4.2, style-inject@0.3.0, sugarss@3.0.3