RFR: 8374293: Jshell throws an error and crashes when using keyword Public

[Lei Zhu]

`jdk.jshell.SourceCodeAnalysis#highlights` should not return overlapping highlights, so use `TreeSet` for deduplication and sorting.

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Commit messages:
 - 8374293: Jshell throws an error and crashes when using keyword Public

Changes: https://git.openjdk.org/jdk/pull/29018/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29018&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8374293
  Stats: 23 lines in 2 files changed: 11 ins; 7 del; 5 mod
  Patch: https://git.openjdk.org/jdk/pull/29018.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29018/head:pull/29018

PR: https://git.openjdk.org/jdk/pull/29018