Group template helper functions, remove Printf, improve template error messages (#23982)

Follow #23328 


Major changes:

* Group the function in `templates/help.go` by their purposes. It could
make future work easier.
* Remove the `Printf` helper function, there is already a builtin
`printf`.
* Remove `DiffStatsWidth`, replace with `Eval` in template
* Rename the `NewTextFuncMap` to `mailSubjectTextFuncMap`, it's for
subject text template only, no need to make it support HTML functions.


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And fine tune template error messages, to make it more friendly to
developers and users.


![image](https://user-images.githubusercontent.com/2114189/230714245-4fd202d1-2b25-41b2-8be5-03c5fee45091.png)


![image](https://user-images.githubusercontent.com/2114189/230714277-66783577-2a03-49d5-8e8c-ceba5e07a2d4.png)

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Co-authored-by: silverwind <me@silverwind.io>

modules/test/context_tests.go

@@ -133,8 +133,8 @@ func (rw *mockResponseWriter) Push(target string, opts *http.PushOptions) error
 
 type mockRender struct{}
 
-func (tr *mockRender) TemplateLookup(tmpl string) *template.Template {
-	return nil
+func (tr *mockRender) TemplateLookup(tmpl string) (*template.Template, error) {
+	return nil, nil
 }
 
 func (tr *mockRender) HTML(w io.Writer, status int, _ string, _ interface{}) error {