refactor: modernize Go primitives

- Replace interface{} with any (Go 1.18) across confluence/api.go,
  macro/macro.go, util/cli.go, util/error_handler.go, includes/templates.go
- Replace sort.SliceStable with slices.SortStableFunc + cmp.Compare (Go 1.21)
  in attachment/attachment.go, consistent with existing slices usage
- Replace fmt.Errorf("%s", msg) with errors.New(msg) in mark.go

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

attachment/attachment.go

@@ -13,7 +13,8 @@ import (
 	"net/url"
 	"path"
 	"path/filepath"
-	"sort"
+	"cmp"
+	"slices"
 	"strconv"
 	"strings"
 
@@ -235,8 +236,8 @@ func CompileAttachmentLinks(markdown []byte, attachments []Attachment) []byte {
 	// attachments/a.jpg
 	// attachments/a.jpg.jpg
 	// so we replace longer and then shorter
-	sort.SliceStable(replaces, func(i, j int) bool {
-		return len(replaces[i]) > len(replaces[j])
+	slices.SortStableFunc(replaces, func(a, b string) int {
+		return cmp.Compare(len(b), len(a))
 	})
 
 	for _, replace := range replaces {