RFR: 8366478: BigDecimal roots

[fabioromano1]

An implementation of `BigDecimal.nthRoot(int, MathContext)`.

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Commit messages:
 - Removed trailing whitespaces
 - Add BigDecimal.nthRoot()

Changes: https://git.openjdk.org/jdk/pull/27148/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27148&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8366478
  Stats: 275 lines in 1 file changed: 91 ins; 116 del; 68 mod
  Patch: https://git.openjdk.org/jdk/pull/27148.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27148/head:pull/27148

PR: https://git.openjdk.org/jdk/pull/27148