feat(build): linter for missing msgid definitions (#7109)

Add a new linter that checks that basic usages (those with an constant string) of the `Tr` function in Go and template files are referring to an existing translation value. Add it to the CI stack but not make it fail yet.

Signed-off-by: Ellen Emilia Anna Zscheile <fogti+devel@ytrizja.de>
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/7109
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Ellen Emilia Anna Zscheile <fogti+devel@ytrizja.de>
Co-committed-by: Ellen Emilia Anna Zscheile <fogti+devel@ytrizja.de>

build/lint-locale-usage/lint-locale-usage_test.go

@@ -0,0 +1,44 @@
+// Copyright 2025 The Forgejo Authors. All rights reserved.
+// SPDX-License-Identifier: MIT
+
+package main
+
+import (
+	"go/token"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+)
+
+func HandleGoFileWrapped(t *testing.T, fname, src string) []string {
+	var ret []string
+	omh := OnMsgidHandler(func(fset *token.FileSet, pos token.Pos, msgid string) {
+		ret = append(ret, msgid)
+	})
+	require.NoError(t, omh.HandleGoFile(fname, src))
+	return ret
+}
+
+func HandleTemplateFileWrapped(t *testing.T, fname, src string) []string {
+	var ret []string
+	omh := OnMsgidHandler(func(fset *token.FileSet, pos token.Pos, msgid string) {
+		ret = append(ret, msgid)
+	})
+	require.NoError(t, omh.HandleTemplateFile(fname, src))
+	return ret
+}
+
+func TestUsagesParser(t *testing.T) {
+	t.Run("go, simple", func(t *testing.T) {
+		assert.EqualValues(t,
+			[]string{"what.an.example"},
+			HandleGoFileWrapped(t, "<g1>", "package main\nfunc Render(ctx *context.Context) string { return ctx.Tr(\"what.an.example\"); }\n"))
+	})
+
+	t.Run("template, simple", func(t *testing.T) {
+		assert.EqualValues(t,
+			[]string{"what.an.example"},
+			HandleTemplateFileWrapped(t, "<t1>", "{{ ctx.Locale.Tr \"what.an.example\" }}\n"))
+	})
+}