Editor: Add text-shadow block support and theme.json presets

[t-hamano]

Backports the server-side portion of Gutenberg PR #79584, which adds full text-shadow support to the typography block supports and theme.json.

Blocks can now opt in via supports.typography.textShadow, and themes can define presets under settings.typography.textShadowPresets (with textShadow and defaultTextShadowPresets to control the UI and default presets). Unlike shadow support, preset values are serialized as a class name (has-{slug}-text-shadow) rather than an inline style, so the Style Engine definition registers both css_vars and classnames.

Notes:

• The Style Engine had no textShadow entry at all, so the whole definition is added here rather than only the css_vars/classnames keys.
• @since tags use 7.2.0 to match the current trunk version.
• theme.json / theme-i18n.json are not touched, since they are copied from the Gutenberg package during the build. The default presets will land with the next package sync.
• The block.json changes for Paragraph and Heading are not included, since those are synced from the Gutenberg package as well.

Trac ticket: https://core.trac.wordpress.org/ticket/65869

Use of AI Tools

AI assistance: Yes
Tool(s): Claude Code
Model(s): Claude Opus 5
Used for: Porting the Gutenberg changes to their Core equivalents. The original Gutenberg implementation and this backport were both reviewed and edited by me.

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