fix(help): only set url for nvim-owned tags
Problem:
1. gx on |nonexistingtag| opens
https://neovim.io/doc/user/helptag.html?nonexistingtag.
2. b:undo_ftplugin doesn't remove url extmarks.
Solution:
1. Check if the tag is defined in a help file in $VIMRUNTIME.
2. Solution: clear namespace for buffer in b:undo_ftplugin.