Clarify PostgreSQL DO support and verify the reported block

[minleejae]

PostgreSQL DO statements are already accepted with Dialect.POSTGRESQL, but the unsupported-grammar guide still labels them unsupported. Correct the procedural-support description and add a usage example showing the required dialect and access to the preserved body literal.

Replace the simplified regression with the full SQL reported in the issue. Verify exact body preservation, including nested SQL, non-ASCII strings and internal semicolons, while statements before and after the block remain separate. Check deparser output and reparsing with unsupported-statement fallback disabled. Include the fixture's standard license header and compare the emitted statement independently of leading comments, so Gradle and Maven exercise the same input.

The body remains an opaque StringValue; parsing or validating its internal procedural statements is deferred. This follows the PostgreSQL DO syntax, which takes a string literal and allows LANGUAGE before or after it. The reported END $$; is valid: the outermost PL/pgSQL END does not require a semicolon. No production parser or AST code changes are needed for this scope.

Closes #1946.

Validation:

• Java 17 ./gradlew check: passed, 6,904 tests, 0 failures/errors, 25 skipped.
• Java 17 mvn --batch-mode --no-transfer-progress verify: passed, 6,886 tests, 0 failures/errors, 25 skipped.
• PostgreSQL 17.11: executed 11 cases both as original input and after JSqlParser deparsing, including both documentation examples, language placement, quoted/escaped/tagged bodies and the full issue SQL. For the issue SQL, created a minimal schema, verified the inserted values, and executed it again to check the already-present branch. The isolated server was stopped afterward.
• Sphinx HTML: builds successfully with no new warnings compared with master; the PostgreSQL example's highlighting warning is removed.