Backport: Add defaultFontSizes option to theme.json

[ajlende]

Backport WordPress/gutenberg#56661.

Adds a defaultFontSizes option to disable showing the default font sizes.

When defaultFontSizes === false the options are removed from the UI.

By default defaultFontSizes is true.

Trac ticket: https://core.trac.wordpress.org/ticket/60284

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[github-actions]

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[youknowriad]

Looks like there's some failing unit tests, can we fix these?

[youknowriad]

What's the status of this backport @ajlende?

[github-actions]

The following accounts have interacted with this PR and/or linked issues. I will continue to update these lists as activity occurs. You can also manually ask me to refresh this list by adding the props-bot label.

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Props ajlende, youknowriad.

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[youknowriad]

Closing as this PR has been reverted in Gutenberg.