RFR: 8302590: Add String.indexOf(int ch, int fromIndex, int toIndex)

[Raffaello Giulietti]

Add an `indexOf()` variant allowing to specify both a lower and an upper bound on the search.

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Commit messages:
 - 8302590: Add String.indexOf(int ch, int fromIndex, int toIndex)

Changes: https://git.openjdk.org/jdk/pull/12600/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12600&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8302590
  Stats: 245 lines in 4 files changed: 218 ins; 2 del; 25 mod
  Patch: https://git.openjdk.org/jdk/pull/12600.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12600/head:pull/12600

PR: https://git.openjdk.org/jdk/pull/12600