Add a failing test case for & within a nested function in a declaration value

[Andarist]

Related to emotion-js/emotion#2943

[Andarist]

\f gets incorrectly appended to & here:

stylis/src/Parser.js

Lines 47 to 49 in bc0b1b0

	if (indexof(characters += replace(delimit(character), '&', '&\f'), '&\f') != -1)
	ampersand = -1
	break

I'd like to conditionally use this replace here - we need to skip it when we are in the context of the property. I've tried to use property for this and while I managed to fix this case I broke some other cases like this one:

stylis/test/Parser.js

Lines 593 to 601 in bc0b1b0

	test('pseudo-selector with & in a string', () => {
	expect(
	stylis(`
	h1:matches(&,[href="https://css-tricks.com?a=1&b=2"]) {
	display: none
	}
	`)
	).to.equal(`h1:matches(.user,[href="https://css-tricks.com?a=1&b=2"]){display:none;}`)
	})

[Andarist]

@thysultan friendly 🏓

[github-actions]

Pull Request Test Coverage Report for Build 673315f00359b3ae65949786d475d88332f8149d-PR-307

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.006%) to 98.849%

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Totals
Change from base Build bc0b1b03f5562eb8b38601e4e05461631a91c400:	0.006%
Covered Lines:	271
Relevant Lines:	273

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💛 - Coveralls

[thysultan]

@Andarist didn't notice this when you opened it, (only the friendly ping): fixed in the head commit, you can merge/release this one.