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>

modules/indexer/code/elasticsearch/elasticsearch.go

@@ -135,7 +135,7 @@ func (b *Indexer) addUpdate(ctx context.Context, batchWriter git.WriteCloserErro
 			Id(id).
 			Doc(map[string]any{
 				"repo_id":    repo.ID,
-				"content":    string(charset.ToUTF8DropErrors(fileContents)),
+				"content":    string(charset.ToUTF8DropErrors(fileContents, charset.ConvertOpts{})),
 				"commit_id":  sha,
 				"language":   analyze.GetCodeLanguage(update.Filename, fileContents),
 				"updated_at": timeutil.TimeStampNow(),