RFR: 8374909: CodeArea: Exception in IME

[Andy Goryachev]

Adds several tests and fixes export range bugs in the `StyledTextModel`.

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Commit messages:
 - lf
 - whitespace
 - fix
 - tests

Changes: https://git.openjdk.org/jfx/pull/2029/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=2029&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374909
  Stats: 242 lines in 7 files changed: 198 ins; 30 del; 14 mod
  Patch: https://git.openjdk.org/jfx/pull/2029.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/2029/head:pull/2029

PR: https://git.openjdk.org/jfx/pull/2029