Create url() helper

[notoriousb1t]

Acceptance criteria:

• Follows spec https://developer.mozilla.org/en-US/docs/Web/CSS/url.
• Should go in src/strings.ts
• Should have at least one test in src/tests/strings.ts
• Signature should be url(val: string): string;

[notoriousb1t]

Added in 9912275f74984dd1c8fdccbd643785cf45d25aed