Merge template functions "dict/Dict/mergeinto" (#23932)

One of the steps in #23328


Before there were 3 different but similar functions: dict/Dict/mergeinto

The code was just copied & pasted, no test.

This PR defines a new stable `dict` function, it covers all the 3 old
functions behaviors, only +160 -171


Future developers do not need to think about or guess the different dict
functions, just use one: `dict`

Why use `dict` but not `Dict`? Because there are far more `dict` than
`Dict` in code already ......

modules/templates/util.go

@@ -0,0 +1,47 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates
+
+import (
+	"fmt"
+	"reflect"
+)
+
+func dictMerge(base map[string]any, arg any) bool {
+	if arg == nil {
+		return true
+	}
+	rv := reflect.ValueOf(arg)
+	if rv.Kind() == reflect.Map {
+		for _, k := range rv.MapKeys() {
+			base[k.String()] = rv.MapIndex(k).Interface()
+		}
+		return true
+	}
+	return false
+}
+
+// dict is a helper function for creating a map[string]any from a list of key-value pairs.
+// If the key is dot ".", the value is merged into the base map, just like Golang template's dot syntax: dot means current
+// The dot syntax is highly discouraged because it might cause unclear key conflicts. It's always good to use explicit keys.
+func dict(args ...any) (map[string]any, error) {
+	if len(args)%2 != 0 {
+		return nil, fmt.Errorf("invalid dict constructor syntax: must have key-value pairs")
+	}
+	m := make(map[string]any, len(args)/2)
+	for i := 0; i < len(args); i += 2 {
+		key, ok := args[i].(string)
+		if !ok {
+			return nil, fmt.Errorf("invalid dict constructor syntax: unable to merge args[%d]", i)
+		}
+		if key == "." {
+			if ok = dictMerge(m, args[i+1]); !ok {
+				return nil, fmt.Errorf("invalid dict constructor syntax: dot arg[%d] must be followed by a dict", i)
+			}
+		} else {
+			m[key] = args[i+1]
+		}
+	}
+	return m, nil
+}