runtime(html): guard against an existing b:undo_ftplugin var

runtime(html): guard against an existing b:undo_ftplugin var

Filetype plugins should not assume they are the only file to execute on
behalf of a buffer's filetype: other filetypes may use them, and
dotted filetypes may cause multiple to run. When this occurs, they
should _build_ on their respective b:undo_ftplugin settings, not
overwrite each other.

For example, when using a dotted filetype wiki.markdown, the wiki
filetype plugins go first. Then, during the markdown filetype plugins,
the HTML plugin's unconditional assignment to b:undo_ftplugin trashes
any data previously stored there by the wiki filetype.

Follow the pattern elsewhere of assigning or appending conditionally.

closes: #18267

Signed-off-by: D. Ben Knoble <ben.knoble+github@gmail.com>
Signed-off-by: Doug Kearns <dougkearns@gmail.com>
Signed-off-by: Christian Brabandt <cb@256bit.org>