[code-reflection] RFR: Sealed interfaces marking operations that model a Java language construct

[Paul Sandoz]

Define two sealed interfaces `JavaExpression` and `JavaStatement`. An operation implementing `JavaExpression` models some Java expression. An operation implementing `JavaStatement` models some Java statement. An operation implementing both models some expression statement e.g., a method invocation expression or statement.

Not all operations implement these interfaces, they instead contribute to modeling Java program behavior and do not model a specific statement or expression. Operations modeling Java patterns are in a separate category.

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Commit messages:
 - Sealed interfaces marking operations associated

Changes: https://git.openjdk.org/babylon/pull/76/files
  Webrev: https://webrevs.openjdk.org/?repo=babylon&pr=76&range=00
  Stats: 153 lines in 2 files changed: 100 ins; 1 del; 52 mod
  Patch: https://git.openjdk.org/babylon/pull/76.diff
  Fetch: git fetch https://git.openjdk.org/babylon.git pull/76/head:pull/76

PR: https://git.openjdk.org/babylon/pull/76