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:
Similar to clearmatches(), it's always necessary to provide a fallback
that allows the user to do a "global reset" when something goes wrong.
Solution:
vim.img.del(math.huge) clears all images.
Use kitty's d=A command to clear all placements in a single
escape sequence rather than N individual deletes, also freeing stored
image data not referenced by the scrollback buffer.
