codegen: Refactor core jennies for reusability, add version-picking metajennies (#58995)

* Add each-major jenny, refactor TS jenny for it

* Introduce LatestJenny, refactor GoTypesJenny for it

pkg/codegen/generators.go

@@ -57,7 +57,6 @@ func StructuredForGen(k kindsys.Structured) *DeclForGen {
 type DeclForGen struct {
 	*kindsys.SomeDecl
 	lin thema.Lineage
-	sch thema.Lineage
 }
 
 // Lineage returns the [thema.Lineage] for the underlying [kindsys.SomeDecl].
@@ -65,10 +64,15 @@ func (decl *DeclForGen) Lineage() thema.Lineage {
 	return decl.lin
 }
 
-// Schema returns the [thema.Schema] that a jenny should operate against, for those
-// jennies that target a single schema.
-func (decl *DeclForGen) Schema() thema.Lineage {
-	return decl.sch
+// ForLatestSchema returns a [SchemaForGen] for the latest schema in this
+// DeclForGen's lineage.
+func (decl *DeclForGen) ForLatestSchema() SchemaForGen {
+	comm := decl.Meta.Common()
+	return SchemaForGen{
+		Name:    comm.Name,
+		Schema:  decl.Lineage().Latest(),
+		IsGroup: comm.LineageIsGroup,
+	}
 }
 
 // SlashHeaderMapper produces a FileMapper that injects a comment header onto
@@ -82,22 +86,28 @@ func SlashHeaderMapper(maingen string) codejen.FileMapper {
 		case ".json", ".yml", ".yaml":
 			return f, nil
 		default:
-			b := new(bytes.Buffer)
-			fmt.Fprintf(b, headerTmpl, filepath.ToSlash(maingen), f.FromString())
-			fmt.Fprint(b, string(f.Data))
-			f.Data = b.Bytes()
+			buf := new(bytes.Buffer)
+			if err := tmpls.Lookup("gen_header.tmpl").Execute(buf, tvars_gen_header{
+				MainGenerator: maingen,
+				Using:         f.From,
+			}); err != nil {
+				return codejen.File{}, fmt.Errorf("failed executing gen header template: %w", err)
+			}
+			fmt.Fprint(buf, string(f.Data))
+			f.Data = buf.Bytes()
 		}
 		return f, nil
 	}
 }
 
-var headerTmpl = `// THIS FILE IS GENERATED. EDITING IS FUTILE.
-//
-// Generated by:
-//     %s
-// Using jennies:
-//     %s
-//
-// Run 'make gen-cue' from repository root to regenerate.
-
-`
+// SchemaForGen is an intermediate values type for jennies that holds both a thema.Schema,
+// and values relevant to generating the schema that should properly, eventually, be in
+// thema itself.
+type SchemaForGen struct {
+	// The PascalCase name of the schematized type.
+	Name string
+	// The schema to be rendered for the type itself.
+	Schema thema.Schema
+	// Whether the schema is grouped. See https://github.com/grafana/thema/issues/62
+	IsGroup bool
+}