RFR: 8303648: Add String.indexOf(String str, int beginIndex, int endIndex)

[Raffaello Giulietti]

As a followup of [JDK-8302590](https://bugs.openjdk.org/browse/JDK-8302590), this issue covers the analogous case for a search of a string rather than a character.

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Commit messages:
 - 8303648: Add String.indexOf(String str, int beginIndex, int endIndex)

Changes: https://git.openjdk.org/jdk/pull/12903/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12903&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8303648
  Stats: 251 lines in 2 files changed: 236 ins; 5 del; 10 mod
  Patch: https://git.openjdk.org/jdk/pull/12903.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12903/head:pull/12903

PR: https://git.openjdk.org/jdk/pull/12903