Preserve BOM in web editor (#28935) (#28959)

Backport #28935 by @silverwind

The `ToUTF8*` functions were stripping BOM, while BOM is actually valid
in UTF8, so the stripping must be optional depending on use case. This
does:

- Add a options struct to all `ToUTF8*` functions, that by default will
strip BOM to preserve existing behaviour
- Remove `ToUTF8` function, it was dead code
- Rename `ToUTF8WithErr` to `ToUTF8`
- Preserve BOM in Monaco Editor
- Remove a unnecessary newline in the textarea value. Browsers did
ignore it, it seems but it's better not to rely on this behaviour.

Fixes: https://github.com/go-gitea/gitea/issues/28743
Related: https://github.com/go-gitea/gitea/issues/6716 which seems to
have once introduced a mechanism that strips and re-adds the BOM, but
from what I can tell, this mechanism was removed at some point after
that PR.

Co-authored-by: silverwind <me@silverwind.io>
(cherry picked from commit b8e6cffd31)

routers/web/repo/editor.go

@@ -166,8 +166,8 @@ func editFile(ctx *context.Context, isNewFile bool) {
 		}
 
 		buf = append(buf, d...)
-		if content, err := charset.ToUTF8WithErr(buf); err != nil {
-			log.Error("ToUTF8WithErr: %v", err)
+		if content, err := charset.ToUTF8(buf, charset.ConvertOpts{KeepBOM: true}); err != nil {
+			log.Error("ToUTF8: %v", err)
 			ctx.Data["FileContent"] = string(buf)
 		} else {
 			ctx.Data["FileContent"] = content