Add semanticsLabel parameter to TextSpan

[jonahwilliams]

Description

This allows customizing the semantics label when using RichText/Spans per span.

Related Issues

Fixes #30636

Tests

I added the following tests:

• recognizers split semantic nodes with text span labels
• semantics configuration handles TextSpans with semanticsLabels
• TextSpan toPlainText
• TextSpan toPlainText with semanticsLabel

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Breaking Change

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[dnfield]

LGTM