fix(assets): avoid splitting , inside base64 value of srcset attribute

[chaejunlee]

fixes #15419

Description

As mentioned in the issue, when parsing srcset, the , inside base64 value was getting split and joined again, breaking the base64 encoding.

The comment of splitSrcSet says that it is handling "data:..." case. But RE itself is only catching single or double quotes. I added additional RE logic to capture the whole base64 values.

Additional context

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What is the purpose of this pull request?

• Bug fix
• New Feature
• Documentation update
• Other

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• Update the corresponding documentation if needed.
• Ideally, include relevant tests that fail without this PR but pass with it.

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[bluwy]

Looks great 👍