Export an object from the CSS-in-JS syntax

[m-allanson]

Which issue, if any, is this issue related to?

This change is a small part of the changes in PR #4796.

Is there anything in the PR that needs further explanation?

The CSS-in-JS syntax was previously being exported as a function instead of an object. This worked because functions can still have properties attached to them. It was a bit confusing though.

This PR updates the syntax to export an object.

After the first commit in this PR, the test should fail like this:

 FAIL  lib/__tests__/syntaxes.test.js
  ● syntaxes › syntax is an object with parse and stringify keys: css-in-js

    expect(received).toMatchObject(expected)

    Matcher error: received value must be a non-null object

    Received has type:  function
    Received has value: [Function syntax]

      10 | describe('syntaxes', () => {
      11 |      it.each(inputs)('syntax is an object with parse and stringify keys: %s', (name, module) => {
    > 12 |              expect(module).toMatchObject({
         |                             ^
      13 |                      parse: expect.any(Function),
      14 |                      stringify: expect.any(Function),
      15 |              });

      at lib/__tests__/syntaxes.test.js:12:18

[hudochenkov]

Looks like this should be fixed in @stylelint/postcss-css-in-js, because it doesn't export an object.

[m-allanson]

A couple of the other syntaxes do the same thing, check out:

stylelint/lib/syntaxes/syntax-markdown.js

Lines 3 to 8 in 33783f1

	module.exports = require('@stylelint/postcss-markdown')({
	scss: require('postcss-scss'),
	less: require('postcss-less'),
	sass: require('postcss-sass'),
	sugarss: require('sugarss'),
	});

stylelint/lib/syntaxes/syntax-html.js

Lines 3 to 8 in 33783f1

	module.exports = require('postcss-html')({
	scss: require('postcss-scss'),
	less: require('postcss-less'),
	sass: require('postcss-sass'),
	sugarss: require('sugarss'),
	});

I think the function export is by design, so it can be configured at require time. e.g. https://github.com/stylelint/postcss-css-in-js/tree/7a28eabd8851114b3394b89c8d510bd1da2e1690#advanced-use-cases

This seemed like the most pragmatic "fix" given that some of the other syntaxes work like this too. What do you think?

[jeddy3]

Thanks.