[lworld] RFR: 8263900: [lworld] Make .default a separate node type in the parser.

[Jesper Steen Møller]

To prepare for making .default a poly expression and to implement further logic, .default-expressions should be split off as a separate AST node.

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Commit messages:
 - Make .default a separate node type in the parser.

Changes: https://git.openjdk.java.net/valhalla/pull/370/files
 Webrev: https://webrevs.openjdk.java.net/?repo=valhalla&pr=370&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8263900
  Stats: 238 lines in 17 files changed: 200 ins; 27 del; 11 mod
  Patch: https://git.openjdk.java.net/valhalla/pull/370.diff
  Fetch: git fetch https://git.openjdk.java.net/valhalla pull/370/head:pull/370

PR: https://git.openjdk.java.net/valhalla/pull/370