Repository state is behind vim-runtime

Despite this repository is being linked to from [kdl.dev](https://kdl.dev/) \> _Editor Support_ \> _vim_, and originally was upstream for [vim-runtime][], it is technically currently not the official upstream repository.

Please see #3 of this repository and [#14956][] on vim/vim for the original inclusion. The newest commit of this repository is from [2023-02-20][] while the version included in vim-runtime is dated [2024-06-16][]. From a [search] of vim/vim's pull requests it appears newer changes have been pulled in without going through a plugin upstream.

The original inclusion came from [jiangyinzuo/vim][] and given how @jiangyinzuo is also listed for kdl in [MAINTAINERS][] one can probably assume that fork might be the most appropriate place to submit patches.

Making contributions in this situation becomes a bit of an administrative nightmare. Downstream tends to prefer all changes to first land and be tested upstream. However if upstream is the vim fork, rather than this repository, validating patches becomes quite fiddly.

One suggestion here would be to resynchronize this repository from vim-runtime and again make it the official upstream. I however assume that is not going to happen.

Instead this issue merely intends to serve as information to anyone stumbling upon this repository, in their quest for better `ft=kdl` support.

I did:

```sh
git submodule add --name vim https://github.com/jiangyinzuo/vim/
git rm ftplugin/kdl.vim indent/kdl.vim syntax/kdl.vim
mkdir -p ftplugin/ indent/ syntax/
ln -s $( pwd )/vim/runtime/ftplugin/kdl.vim ftplugin
ln -s $( pwd )/vim/runtime/indent/kdl.vim indent
ln -s $( pwd )/vim/runtime/syntax/kdl.vim syntax
git add ftplugin/kdl.vim indent/kdl.vim syntax/kdl.vim
git commit -m "tmp: work towards jiangyinzuo's repo"
```

That's not super-convenient, but it kind of allows you to work on the files of this plugin while having it installed under `~/.vim/pack/`.

[#14956]: https://github.com/vim/vim/pull/14956
[vim-runtime]: https://github.com/vim/vim/tree/master/runtime
[jiangyinzuo/vim]: https://github.com/jiangyinzuo/vim/tree/feat-kdl
[MAINTAINERS]: https://github.com/vim/vim/blob/d3d54f473f6f42372fce8b3ee8eb8f1f3661af7e/.github/MAINTAINERS#L216
[2023-02-20]: https://github.com/imsnif/kdl.vim/commit/b84d7d3a15d8d30da016cf9e98e2cfbe35cddee5
[2024-06-16]: https://github.com/vim/vim/blob/834b8d218f1db92e587ecd449e5ce88b41fbe256/runtime/syntax/kdl.vim#L5
[search]: https://github.com/vim/vim/pulls?q=is%3Apr+kdl+is%3Aclosed

Start by comparing this repository's ftplugin/kdl.vim, indent/kdl.vim, and syntax/kdl.vim with the versions in vim-runtime and jiangyinzuo/vim. Review vim/vim#14956 and the linked pull-request search to establish which repository is upstream; done would require an agreed, documented upstream and synchronization path.

Written by the indexing model from the issue text.