feat(analysis): accept ON CONFLICT upserts on inserts

[omer-cengel]

Summary

Inserts can now be written as PostgreSQL upserts. INSERT ... ON CONFLICT (columns) DO NOTHING and ON CONFLICT (columns) DO UPDATE SET ... compile on :exec and returning inserts, with every placeholder typed and bound once per textual occurrence, so the common insert-or-update idiom no longer needs a workaround. Conflict forms the analyzer cannot describe are rejected on every insert instead of passing through unchecked on :exec.

Changes

• Accept a conflict target that is a parenthesized list of plain column names of the inserted table, each resolved against the schema.
• Accept DO NOTHING, which binds only the VALUES row.
• Accept DO UPDATE SET assignments analyzed exactly like UPDATE assignments: one column at a time, where a value is a placeholder typed by its column, a cast placeholder typed by its cast, EXCLUDED.column (resolved against the inserted table), or any other expression passed through as written.
• Bind DO UPDATE placeholders after the VALUES placeholders in textual order. A repeated named placeholder binds at each occurrence.
• Allow the same upsert with RETURNING, including RETURNING * resolving the table's shared row record.
• Reject, on :exec and returning inserts alike, a missing target, ON CONFLICT ON CONSTRAINT, an expression, collated, or operator-class target element, a target predicate, and DO UPDATE ... WHERE. Reject unknown target, assigned, or EXCLUDED columns.
• Add analyzer tests for the accepted and rejected forms, and a PostgreSQL test that runs both actions, with and without RETURNING, over a new row and then a conflicting one.
• Document the upsert shape, binding order, DO NOTHING ... RETURNING cardinality, and the rejected forms.

Scope and non-goals

• sqlcj does not check that the conflict target matches a unique index; PostgreSQL does.
• An EXCLUDED reference nested inside a larger expression is not resolved, the same as other computed write values.
• MERGE, ON DUPLICATE KEY, multi-row and SELECT-sourced inserts are unchanged and unsupported.
• SQL parameter compilation, code generation, and the runtime are unchanged.