feat: support MERGE ... WHEN NOT MATCHED BY TARGET / BY SOURCE (#2421)

[fudianchn]

Problem

BigQuery and SQL Server allow the MERGE ... WHEN NOT MATCHED clause to be qualified with BY TARGET (default, allows INSERT) or BY SOURCE (allows UPDATE/DELETE). JSQLParser currently rejects it as an UnsupportedStatement (#2421):

MERGE INTO target_table AS tt
USING (SELECT key, field FROM source_table) AS st ON tt.key = st.key
WHEN NOT MATCHED BY TARGET THEN INSERT (key, field) VALUES (st.key, st.field)
WHEN NOT MATCHED BY SOURCE THEN DELETE;

The grammar's MergeWhenNotMatched had no BY branch and was hard-wired to INSERT.

Change

• Grammar (JSqlParserCC.jjt): add an optional [BY TARGET|SOURCE] to WHEN NOT MATCHED, and let that branch produce INSERT, UPDATE, or DELETE (extracted MergeInsertClause helper so all three branches stay simple, mirroring MergeWhenMatched). Standard SQL WHEN [NOT] MATCHED (no BY) is unchanged.
• Model: new MergeSide { TARGET, SOURCE } enum + a side field on MergeInsert/MergeUpdate/MergeDelete, rendered by both toString() and the MergeDeParser visitor.
• TARGET/SOURCE are matched as identifiers (S_IDENTIFIER) rather than reserved keywords, so existing SQL that uses them as column/table names is unaffected.

Verification

• New tests in MergeTest cover BY TARGET (INSERT) + BY SOURCE (DELETE), and BY SOURCE ... THEN UPDATE with an AND predicate — each with a round-trip assertSqlCanBeParsedAndDeparsed plus AST assertions on the side field.
• Full suite: 4659 tests, 0 failures. checkstyle / pmd / license:check-file-header clean.
• Backward compatible: existing WHEN MATCHED ... UPDATE/DELETE and WHEN NOT MATCHED ... INSERT tests keep passing (23 pre-existing Merge tests unaffected).

Fixes #2421.