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?

• [x] Bug fix
• [ ] New Feature
• [ ] Documentation update
• [ ] Other

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• [x] Read the Contributing Guidelines, especially the Pull Request Guidelines.
• [x] Check that there isn't already a PR that solves the problem the same way to avoid creating a duplicate.
• [x] Provide a description in this PR that addresses what the PR is solving, or reference the issue that it solves (e.g. fixes #123).
• [x] Update the corresponding documentation if needed.
• [x] Ideally, include relevant tests that fail without this PR but pass with it.

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