[Good First Issue] Strongly type Field.type as a FieldType union

[devthedevil]

Summary

Adds a FieldType union type covering all PlanetScale / Vitess column types and uses it for Field.type and the types map on ExecutedQuery.

Changes

• Export new FieldType union sourced from vitessio/vitess go/sqltypes/type.go
• Change Field.type from string → FieldType
• Change type Types from Record<string, string> → Record<string, FieldType>
• The | (string & {}) fallback keeps the union open for future server-side types

Related Issue

Closes #141

Testing

Purely additive type-level change — no runtime behaviour changes. Verify with npm test.