Refactor html templates

This commit entirely refactors the use of html templates. Instead of
inheriting from a shared template, we insert the results into a base
template.

internal/dis/component/server/templating/parse.go

@@ -0,0 +1,66 @@
+package templating
+
+import (
+	"html/template"
+	"reflect"
+
+	"github.com/tkw1536/goprogram/lib/reflectx"
+	"golang.org/x/exp/slices"
+)
+
+// Parsed represents a parsed template that takes as argument a context of type C.
+type Parsed[C any] struct {
+	// does the context type an embed of the runtime flags type?
+	hasRuntimeFlagsEmbed bool
+
+	tpl   *template.Template // parsed template
+	funcs []FlagFunc         // optionally concfigured functions.
+}
+
+// Parse parses a template with the given name and source.
+// If base is not nil, every template associated with the base template is copied into the given template.
+// Functions will be applied on creation time to represent the context for the given template.
+func Parse[C any](name string, source []byte, base *template.Template, funcs ...FlagFunc) Parsed[C] {
+	tp := reflectx.TypeOf[C]()
+
+	// determine if we have an embedded field in the struct
+	var hasEmbed bool
+	if tp.Kind() == reflect.Struct {
+		field, ok := tp.FieldByName(runtimeFlagsName)
+		if ok {
+			hasEmbed = field.Anonymous
+		}
+	}
+
+	// create a new template, and optionally inherit from the base template
+	new := template.New(name)
+	if base != nil {
+		for _, tree := range base.Templates() {
+			root := tree.Tree.Copy()
+			new.AddParseTree(tree.Name(), root)
+		}
+	}
+
+	return Parsed[C]{
+		hasRuntimeFlagsEmbed: hasEmbed,
+		tpl:                  template.Must(new.Parse(string(source))),
+		funcs:                funcs,
+	}
+}
+
+// Prepare prepares this template to be used with the given templating.
+func (p *Parsed[C]) Prepare(templating *Templating, funcs ...FlagFunc) *Template[C] {
+	pcopy := *p // make a copy of p!
+
+	wrap := Template[C]{
+		templating: templating,
+
+		p: &pcopy,
+	}
+
+	// copy the functions!
+	pcopy.funcs = slices.Clone(pcopy.funcs)
+	pcopy.funcs = append(wrap.p.funcs, funcs...)
+
+	return &wrap
+}