Adds placeholders to AttributedText (Resolves #2440) #2832

Job	Run time
analyze_super_text_layout	58s
test_attributed_text	20s
test_goldens_super_editor	1m 24s
analyze_attributed_text	16s
test_goldens_super_text_layout	1m 3s
test_linux	4m 37s
test_mac	5m 1s
test_super_editor_markdown	1m 5s
test_super_editor_quill	1m 0s
test_super_text_layout	1m 2s
test_windows	6m 1s
	22m 47s

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