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.

refactor(lua): add integer coercion helpers

Add vim._tointeger() and vim._ensure_integer(), including optional base
support, and switch integer-only tonumber()/assert call sites in the Lua
runtime to use them.

This also cleans up related integer parsing in LSP, health, loader, URI,
tohtml, and Treesitter code.

supported by AI

Problem:
In autocmd examples, using "args" as the event-object name is vague and
may be confused with a user-command.

Solution:
Use "ev" as the conventional event-object name.

This function allows the Nvim core to write arbitrary data to a TTY
connected to a UI's stdout.

Type fixes caught by emmylua

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.

When a plugin registers a TermRequest handler there is currently no way
for the handler to know where the terminal's cursor position was when
the sequence was received. This is often useful information, e.g. for
OSC 133 sequences which are used to annotate shell prompts.

Modify the event data for the TermRequest autocommand to be a table
instead of just a string. The "sequence" field of the table contains the
sequence string and the "cursor" field contains the cursor
position when the sequence was received.

To maintain consistency between TermRequest and TermResponse (and to
future proof the latter), TermResponse's event data is also updated to
be a table with a "sequence" field.

BREAKING CHANGE: event data for TermRequest and TermResponse is now a
table