Stop slugs being cast to ints when theme is exported

[glendaviesnz]

The new spacing preset slugs get cast to ints when a theme is exported as reported in WordPress/gutenberg#44546. This PR makes sure they are cast back to strings if PHP converts them to ints when they are used as array keys.

Trac ticket: https://core.trac.wordpress.org/ticket/56684

Testing

• In a site with the 2023 theme set export the theme from the site editor
• Make sure the exported theme.json has the settings.spacingScale slugs as strings
• Check that the spacing presets still work as expected in post editor, site editor and frontend

[dream-encode]

Merged into core in https://core.trac.wordpress.org/changeset/54363.