Refactor of outputting only duotone css and svg needed for rendered blocks

[jeryj]

I'm sorry this is in one giant commit. The general idea is:

• On wp_loaded
  • save all duotone presets defined in merged global styles
  • save all block names and their corresponding duotone slug info
• on block render, check if the block:
  • is one of the blocks using duotone via global styles
  • has any defined duotone styles
  • if so, add it to the duotone_output array to have its duotone info output

Thanks to @ajlende for co-piloting this.

[github-actions]

Flaky tests detected in 8cdfaec.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/4266725721
📝 Reported issues:

• [Flaky Test] should handle Enter across blocks #39763 in specs/editor/various/multi-block-selection.test.js
• [Flaky Test] should not allow blocks to be inserted in inner blocks #44341 in specs/editor/plugins/cpt-locking.test.js
• [Flaky Test] should paste list contents into paragraph #46718 in specs/editor/various/rich-text.test.js
• [Flaky Test] should write over selection #39938 in specs/editor/various/multi-block-selection.test.js