Problem:
The "tohtml" plugin is loaded by default.
Solution:
- Move it to `pack/dist/opt/nvim.tohtml/`, it is an "opt-in" plugin now.
- Document guidelines.
- Also revert the `plugin/` locations of `spellfile.lua` and `net.lua`.
That idea was not worth the trouble, it will be too much re-education
for too little gain.
Problem: confusing that there is the tag `undo-tree` (the Vim
implementation) and `undotree` (the Lua plugin for visualization).
Solution: rename tag to undotree-plugin. Mention the plugin in the docs of
|undotree|.
Problem:
Difference in highlight color between normal lines and extmark lines is
distracting and conveys little information.
Solution:
Set extmarks highlight to the Normal group.
