RFR: 8370902: RichTextArea: migrate to new text APIs

[Andy Goryachev]

Updates the `RichTextArea` code to use the new text layout APIs delivered in https://bugs.openjdk.org/browse/JDK-8341670

Also:

- updates the "demo" model in the `RichTextAreaDemoApp` to show underline/strikethrough in the presence of `TextFlow` borders

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Commit messages:
 - 2026
 - Merge branch 'master' into 8370902.new.api
 - Merge branch 'master' into 8370902.new.api
 - new api

Changes: https://git.openjdk.org/jfx/pull/2012/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2012&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370902
  Stats: 66 lines in 5 files changed: 14 ins; 26 del; 26 mod
  Patch: https://git.openjdk.org/jfx/pull/2012.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2012/head:pull/2012

PR: https://git.openjdk.org/jfx/pull/2012