Add stylebook path to exceptions for classic themes in site editor.

[tellthemachines]

Trac ticket: https://core.trac.wordpress.org/ticket/62509

Syncs changes from WordPress/gutenberg#66851 and WordPress/gutenberg#68174 to core.

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This Pull Request is for code review only. Please keep all other discussion in the Trac ticket. Do not merge this Pull Request. See GitHub Pull Requests for Code Review in the Core Handbook for more details.

[github-actions]

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