fix implementation of IEngineCore functions in Engine struct

[ip75]

see #299 for description

[gaby]

@ReneWerner87 this is a breaking change, right? 🤔

[ReneWerner87]

yes should be, because we lost the real engine type
but thats the only solution currently without a self keyword

[ReneWerner87]

see
https://medium.com/@mier85/self-referencing-interfaces-in-golang-1-18-bcd6b5701992
https://stackoverflow.com/questions/41881113/interface-that-returns-its-self-cloneable
golang/go#28254

[ip75]

see https://medium.com/@mier85/self-referencing-interfaces-in-golang-1-18-bcd6b5701992 https://stackoverflow.com/questions/41881113/interface-that-returns-its-self-cloneable golang/go#28254

i've gave up to do it with generics
https://go.googlesource.com/proposal/+/refs/heads/master/design/43651-type-parameters.md#no-parameterized-methods

I've made workaround solution with stub struct

var fs embed.FS

func NewTemplatesEngine() template.IEngine {
	mustache := mustache.NewFileSystem(http.FS(fs), ".mustache")
	return &StubEngine{Engine: mustache}
}


type StubEngine struct {
	*mustache.Engine
}

func (e *StubEngine) AddFunc(name string, fn interface{}) template.IEngineCore {
	e.Engine.AddFunc(name, fn)
	return e
}
func (e *StubEngine) AddFuncMap(m map[string]interface{}) template.IEngineCore {
	e.Engine.AddFuncMap(m)
	return e
}
func (e *StubEngine) Debug(enabled bool) template.IEngineCore {
	e.Engine.Debug(enabled)
	return e
}
func (e *StubEngine) Delims(left, right string) template.IEngineCore {
	e.Engine.Delims(left, right)
	return e
}
func (e *StubEngine) FuncMap() map[string]interface{} {
	return e.Engine.FuncMap()
}
func (e *StubEngine) Layout(key string) template.IEngineCore {
	e.Engine.Layout(key)
	return e
}
func (e *StubEngine) Reload(enabled bool) template.IEngineCore {
	e.Engine.Reload(enabled)
	return e
}