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_test.go

@@ -0,0 +1,43 @@
+// Copyright 2023 The Gitea Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package templates
+
+import (
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+)
+
+func TestDict(t *testing.T) {
+	type M map[string]any
+	cases := []struct {
+		args []any
+		want map[string]any
+	}{
+		{[]any{"a", 1, "b", 2}, M{"a": 1, "b": 2}},
+		{[]any{".", M{"base": 1}, "b", 2}, M{"base": 1, "b": 2}},
+		{[]any{"a", 1, ".", M{"extra": 2}}, M{"a": 1, "extra": 2}},
+		{[]any{"a", 1, ".", map[string]int{"int": 2}}, M{"a": 1, "int": 2}},
+		{[]any{".", nil, "b", 2}, M{"b": 2}},
+	}
+
+	for _, c := range cases {
+		got, err := dict(c.args...)
+		if assert.NoError(t, err) {
+			assert.EqualValues(t, c.want, got)
+		}
+	}
+
+	bads := []struct {
+		args []any
+	}{
+		{[]any{"a", 1, "b"}},
+		{[]any{1}},
+		{[]any{struct{}{}}},
+	}
+	for _, c := range bads {
+		_, err := dict(c.args...)
+		assert.Error(t, err)
+	}
+}