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)

templates/repo/editor/edit.tmpl

@@ -38,8 +38,7 @@
 						data-url="{{.Repository.Link}}/markup"
 						data-context="{{.RepoLink}}"
 						data-previewable-extensions="{{.PreviewableExtensions}}"
-						data-line-wrap-extensions="{{.LineWrapExtensions}}">
-{{.FileContent}}</textarea>
+						data-line-wrap-extensions="{{.LineWrapExtensions}}">{{.FileContent}}</textarea>
 					<div class="editor-loading is-loading"></div>
 				</div>
 				<div class="ui bottom attached tab segment markup" data-tab="preview">