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_columns.go

@@ -0,0 +1,21 @@
+package dbx
+
+import "github.com/go-jet/jet/v2/mysql"
+
+// NormalCols processes a list of columns and strips out any that implement any of
+// ColumnTimestamp, ColumnTime, or ColumnDate.
+func NormalCols[CL ColumnList](cols ...Column) CL {
+	res := make(CL, 0)
+
+	for _, col := range cols {
+		switch col.(type) {
+		case mysql.ColumnTimestamp, // = postgres.ColumnTimestamp
+			mysql.ColumnTime, // = postgres.ColumnTime
+			mysql.ColumnDate: // = postgres.ColumnDate
+		default:
+			res = append(res, col)
+		}
+	}
+
+	return res
+}