Fix CSS selector id parsing

[ydah]

Fix CSS selector id parsing so Capybara/SpecificFinders only autocorrects supported simple id selectors.

Previously CssSelector.id removed every # in the selector and trimmed matcher suffixes with a broad regexp. That meant unsupported CSS identifier syntax such as multiple id selectors, escaped #, quoted values, or Unicode escapes could still be treated as a safe id and produce an incorrect find_by_id autocorrection.

This changes id detection to a strict anchored pattern for simple ids and the existing supported escapes, and skips the offense when parsing is unsupported.

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