Problem: Argument type mismatches are excluded from EmmyLua checks.

Solution: Correct runtime annotations and calls, document local checker
workarounds, and enable the diagnostic.

AI-assisted

Problem:
The magic globals `it`, `describe`, etc., are more trouble than they are
worth.

- Hooking into `after_each` requires `getfenv()` hacks.
- They confuse luals/emmylua, because the top-level `.luarc.json` isn't
  merged with `test/.luarc.json` (apparently a luals limitation?)
- They totally defeat discoverability because the user just has to
  "know" about the various magic symbols.

So they harm DX, which means they serve no purpose at all.

Solution:
- Expose the test API from `testutil`, so tests can call `t.it()`,
  `t.describe()`, etc., in the conventional way.
- Drop `getfenv()` hacks.
- Drop the `setfenv()` injection in `load_chunk`.
- Drop `test/_meta.lua`.

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:
Font rendering and kerning are subpar in GUIs.

Solution:
Set default 'guifont' based on common CSS fonts per:
https://github.com/system-fonts/modern-font-stacks#monospace-code

Problem:
Running :TOhtml with a file containing modeline may generate an invalid modeline in the output.

Solution:
Add `<!-- vim: set nomodeline: -->` to the output.
Use vi-compatible modeline format ("set foo:"), to avoid the trailing `-->` being treated as part of the modeline.

Before calling "attach" a screen object is just a dummy container for
(row, col) values whose purpose is to be sent as part of the "attach"
function call anyway.

Just create the screen in an attached state directly. Keep the complete
(row, col, options) config together. It is still completely valid to
later detach and re-attach as needed, including to another session.