RFR: 8370140: RichTextArea: line endings

[Andy Goryachev]

Adds control of line endings (newline separators) in `StyledTextModel`, `RichTextArea`, and `CodeArea`.

The impacted areas are:
- saving to plain text
- copying to plain text

This feature is implemented as a regular field in the `StyledTextModel` (since it is ultimately an attribute of the model), and as a property in the `CodeArea`.

-------------

Commit messages:
 - tests
 - line endings

Changes: https://git.openjdk.org/jfx/pull/1944/files
  Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1944&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8370140
  Stats: 263 lines in 13 files changed: 234 ins; 9 del; 20 mod
  Patch: https://git.openjdk.org/jfx/pull/1944.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1944/head:pull/1944

PR: https://git.openjdk.org/jfx/pull/1944