feature: implement DevMigrator closes #283

[g105b]

New features:

• gt migrate --dev runs normal migrations, then applies query/_migration/dev files tracked separately in the _migration_dev table.
• gt migrate --dev-merge runs normal migrations, then renumbers and moves applied dev migrations into the canonical migration directory and records them as already applied in _migration.

Key changes are in the new DevMigrator, Migrator and ExecuteCommand. I've split out the Migrator class into an abstract base class so both Migrator and DevMigrator can share the functionality.

[codacy-production]

Up to standards ✅

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🟢 Metrics 106 complexity · 0 duplication

Metric	Results
Complexity	106
Duplication	0

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[g105b]

This latest change adds per-statement progress tracking, to allow for partial migration success.

@pixeldrift64 and I are working together on a busy production app, and the database struggled to add a new column in amongst heavy user traffic, but half of the migration did succeed. This change accommodates for this, so a partial success will be recorded, and the next migration will resume from where the current migration succeeded.

Integrity checks are still applied for the whole files, so if a partially applied migration is edited before it resumes, it will correctly fail with a migration integrity error instead of continuing with changed SQL.

Both _migration and _migration_dev tables are upgraded automatically to add the new progress column when migrations run, so existing production installations can adopt the featire without a manual schema change.

Feedback is welcome while I give this more thought.