Generate MySQL REPLACE statements for replacement prefixes

[ac1982]

MySQL REPLACE is a statement, not an INSERT modifier. csvsql --prefix REPLACE --insert currently emits INSERT REPLACE INTO ... and fails with MySQL error 1064.

Use a separate SQLAlchemy Insert subclass for MySQL replacement writes. Its dialect compiler retains SQLAlchemy's column quoting and bound parameters while changing the leading keyword to REPLACE. Other modifiers, ordinary inserts, chunking, and SQLite's OR REPLACE keep their existing paths; the caller's prefix list is not modified.

Fixes #44.

Validation:

• Full agate-sql suite on SQLAlchemy 1.4.54 and 2.1.4: 20 passed, 8 subtests passed. Six new replacement cases fail on the base revision.
• Real MySQL 8.4.11 on both SQLAlchemy versions: ordinary/chunked replacements, LOW_PRIORITY, unchanged INSERT IGNORE, bound values containing INSERT REPLACE, and caller-controlled rollback pass.
• Actual csvsql command reproduces error 1064 before the fix and persists the expected replacement rows afterwards. csvsql tests: 22 passed, 1 skipped, 10 subtests passed.
• flake8, isort, check-manifest, and Sphinx documentation build pass.

Local validation used Linux/Python 3.12. The full OS/Python CI matrix was not run. This change leaves transaction ownership unchanged: csvsql starts and commits its own transaction; the separate owned-connection issue in #68 is independent.

AI-assisted implementation and validation.