CSS Parser: Handle string with semi-colon in custom properties.

[cduez]

Strings are not parsed correctly for custom properties which makes the following CSS raise an Unterminated string: ";" error:

:root { 
  --custom: 'data:text/plain;base64,SGVsbG8sIFdvcmxkIQ==';
}

According to the spec, we should accept semi-colon as long as they are not at the top level.

The allowed syntax for custom properties is extremely permissive. The production matches any sequence of one or more tokens, so long as the sequence does not contain bad-string-token, bad-url-token, unmatched )-token, ]-token, or }-token, or top-level semicolon-token tokens or delim-token tokens with a value of "!".

Extract from: https://drafts.csswg.org/css-variables/#syntax

I was only able to reproduce with tailwindcss v4, the previous version seems to support this. This issue is mitigated by the fact that even if you want to use a data URL in a custom property, you would need to wrap the value in a url() anyway:

:root { 
  --my-icon-url: url('data:image/svg+xml;base64,...==');
}

.icon {
  background-image: var(--my-icon-url);
}

Which works perfectly fine with the current/latest version (v4.1.8).

The fix suggested is to share the same code between regular property and custom property when it comes to detect that the value is a string starting with a SINGLE_QUOTE or DOUBLE_QUOTE. I have moved the existing code in a findEndStringIdx which returns the position of the ending single/double quote.