refactor: split dbx package into domain files

Reorganize monolithic dbx.go and utility.go into focused files by
concern, add package doc.go, and gitignore coverage.out.

Co-authored-by: Cursor <cursoragent@cursor.com>

jet_types.go

@@ -0,0 +1,24 @@
+package dbx
+
+import (
+	"github.com/go-jet/jet/v2/mysql"
+	"github.com/go-jet/jet/v2/postgres"
+)
+
+// Column is a union type for mysql.Column and postgres.Column
+type Column interface {
+	mysql.Column
+	postgres.Column
+}
+
+// ColumnList is a union type for mysql.ColumnList and postgres.ColumnList
+type ColumnList interface {
+	mysql.ColumnList
+	postgres.ColumnList
+}
+
+// BoolExpression is a union type for mysql.BoolExpression and postgres.BoolExpression
+type BoolExpression interface {
+	mysql.BoolExpression
+	postgres.BoolExpression
+}