feat!: support parseable URL values

[charliecreates]

This PR contains:

• bugfix
• feature
• refactor
• tests
• documentation
• metadata

Breaking Changes?

• yes
• no

If yes, please describe the breakage.

The value of isUrl is now determined by different conditions, which could break consumers downstream if relying on older behavior and value of isUrl.

Please Describe Your Changes

• Add support for parseable URL values.

Resolves #157

[shellscape]

@CharlieHelps use the pull request template

[charliecreates]

Done — I updated the PR body to use the repository pull request template and kept the Resolves #157 linkage.

[shellscape]

I see no tests for the myriad of url('...') variants. And since url is producing a Word, the tests should go into the word tests.

[shellscape]

@gwynne I was able to get a review in before I had to step away. please review this when able

[gwynne]

Other than the incomplete test coverage, the rest looks correct to me, and a quick local test shows that it fully covers my use case.

[gwynne]

These are both mislabeled and incomplete. There are three types of meaningful URL—protocol-absolute, file-absolute, and file-relative—and valid inputs can include both query strings (e.g. ?a=b) and fragment identifiers (e.g. #foo). Procotol-relative URLs, while they did historically exist, are not considered valid by modern browsers, and thus should not be considered IMO. Here is a more comprehensive test matrix:

Suggested change

	{ fixture: 'url(https://example.com/image.png)', label: 'unquoted absolute URL' },
	{ fixture: "url('https://example.com/image.png')", label: 'single-quoted absolute URL' },
	{ fixture: 'url("https://example.com/image.png")', label: 'double-quoted absolute URL' },
	{ fixture: 'url(/images/image.png)', label: 'unquoted relative URL' },
	{ fixture: "url('/images/image.png')", label: 'single-quoted relative URL' },
	{ fixture: 'url("/images/image.png")', label: 'double-quoted relative URL' },
	{ fixture: 'url(//cdn.example.com/image.png)', label: 'protocol-relative URL' },
	{ fixture: 'url()', label: 'empty URL value' }
	{ fixture: 'url(https://example.com/image.png)', label: 'unquoted full URL' },
	{ fixture: 'url(/images/image.png)', label: 'unquoted absolute file path' },
	{ fixture: 'url(images/image.png)', label: 'unquoted relative file path' },
	{ fixture: 'url(./images/image.png)', label: 'unquoted relative file path with leading ./' },
	{ fixture: "url('https://example.com/image.png')", label: 'single-quoted full URL' },
	{ fixture: "url('/images/image.png')", label: 'single-quoted absolute file path' },
	{ fixture: "url('images/image.png')", label: 'single-quoted relative file path' },
	{ fixture: "url('./images/image.png')", label: 'single-quoted relative file path with leading ./' },
	{ fixture: 'url("https://example.com/image.png")', label: 'double-quoted full URL' },
	{ fixture: 'url("/images/image.png")', label: 'double-quoted absolute file path' },
	{ fixture: 'url("images/image.png")', label: 'double-quoted relative file path' },
	{ fixture: 'url("./images/image.png")', label: 'double-quoted relative file path with leading ./' },
	{ fixture: "url('https://example.com/image.png?1234567890#abcdef')", label: 'single-quoted full URL with query and fragment' },
	{ fixture: "url('/images/image.png?1234567890#abcdef')", label: 'single-quoted absolute file path with query and fragment' },
	{ fixture: "url('images/image.png?1234567890#abcdef')", label: 'single-quoted relative file path with query and fragment' },
	{ fixture: "url('./images/image.png?1234567890#abcdef')", label: 'single-quoted relative file path with leading ./, query, and fragment' },
	{ fixture: 'url("https://example.com/image.png?1234567890#abcdef")', label: 'double-quoted full URL with query and fragment' },
	{ fixture: 'url("/images/image.png?1234567890#abcdef")', label: 'double-quoted absolute file path with query and fragment' },
	{ fixture: 'url("images/image.png?1234567890#abcdef")', label: 'double-quoted relative file path with query and fragment' },
	{ fixture: 'url("./images/image.png?1234567890#abcdef")', label: 'double-quoted relative file path with leading ./, query, and fragment' },
	{ fixture: 'url()', label: 'empty URL value' },
	{ fixture: "url('')", label: 'empty single-quoted URL value' },
	{ fixture: 'url("")', label: 'empty double-quoted URL value' }