Problem:
Coordinates-to-position conversion for the finish column in
`vim.hl.range()` checks the start column. This results in an incorrect
finish column when only one of the column coordinates is `vim.v.maxcol`.

Solution:
Check the finish column when converting the finish position. Update the
existing screen test so that a `vim.v.maxcol` finish highlights the
end-of-line marker, matching the existing `-1` behavior.

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:
When `vim.hl.range(0, …, { timeout = N })` is called, the deferred
`range_hl_clear` captures `buf=0`, which resolves to an arbitrary
"current buffer" at timeout. This may cause a stale highlight that never
gets cleared.

Solution:
Resolve `buf=0` explicitly, before `range_hl_clear` captures it.

Problem:
vim.hl.on_yank() only works for TextYankPost, not TextPutPost.

Solution:
Introduce hl_op().
Deprecate on_yank().

Problem:
nvim_buf_del_extmark error if buffer is destroyed before timer stops

Solution:
check nvim_buf_is_valid.

Problem: Currently vim.hl.range only allows one timed highlight.
Creating another one, removes the old one.

Solution: vim.hl.range now returns a timer and a function. The timer
keeps track of how much time is left in the highlight and the function
allows you to clear it, letting the user decide what to do with old
highlights.

Problem: yi' don't highlight last character since
https://github.com/neovim/neovim/commit/8ce504820af04194a41acbe1f4c61cf12bd5feb5.

Solution: Always use `opts.inclusive=true`, since calculation of `"]`
(`b_op_end`) have taken `inclusive` into account.