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:
this crashes neovim:

ondska = setmetatable({}, {__tostring = error})
error(ondska)

solution:

make it not crash neovim

- Problem: It's not clear for new plugin developers that `:help` uses
  a help-tags file for searching the docs, generated by `:helptags`.
  - Solution: Hint to the |:helptags| docs for regenerating the tags
    file for their freshly written documentation.

Co-authored-by: Yochem van Rosmalen <git@yochem.nl>

Problem:
decorations_spec.lua, float_spec.lua, multigrid_spec.lua are not
auto-formatted.

Solution:
Add a special `formatlua2` cmake target, which invokes `stylua` with
an alternative `.stylua2.toml` config.

"Error" in error messages is redundant. Just provide the context, don't
say "Error ...".

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.

In the api_info() output:

    :new|put =map(filter(api_info().functions, '!has_key(v:val,''deprecated_since'')'), 'v:val')
    ...

    {'return_type': 'ArrayOf(Integer, 2)', 'name': 'nvim_win_get_position', 'method': v:true, 'parameters': [['Window', 'window']], 'since': 1}

The `ArrayOf(Integer, 2)` return type didn't break clients when we added
it, which is evidence that clients don't use the `return_type` field,
thus renaming Dictionary => Dict in api_info() is not (in practice)
a breaking change.