Preserve BOM in web editor (#28935)

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.

routers/web/repo/view.go

@@ -303,7 +303,7 @@ func renderReadmeFile(ctx *context.Context, subfolder string, readmeFile *git.Tr
 		return
 	}
 
-	rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc))
+	rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc), charset.ConvertOpts{})
 
 	if markupType := markup.Type(readmeFile.Name()); markupType != "" {
 		ctx.Data["IsMarkup"] = true
@@ -492,7 +492,7 @@ func renderFile(ctx *context.Context, entry *git.TreeEntry) {
 			break
 		}
 
-		rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc))
+		rd := charset.ToUTF8WithFallbackReader(io.MultiReader(bytes.NewReader(buf), dataRc), charset.ConvertOpts{})
 
 		shouldRenderSource := ctx.FormString("display") == "source"
 		readmeExist := util.IsReadmeFileName(blob.Name())