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:
After 25439dad7e2b, `opts` is optional, but not params that follow it.

Solution:
- Update `gen_api_dispatch.lua`, docs, tests.
- Enhance the `nvim_get_api_info()` api-metadata to include a boolean
  flag indicating whether a parameter is optional:
  ```
  [type, name, optional]
  ```

Note: Currently, optional params are assumed to be Dict/Array. Since we
don't yet have any use-cases of optional params following `opts`, the
build just checks this assumption and we can deal with it later if we
ever care to:

    nvim_get_hl: optional param "foo" has type "Integer" but (currently)
    we assume Dict/Array

Problem:
|api-contract| allows adding an optional trailing `opts` parameter, but
the dispatch generator still requires every parameter, so `opts` cannot
be omitted.

Solution:
Treat a trailing `Dict(...) *opts` parameter as optional in the generated
RPC and Lua dispatch wrappers.

Co-authored-by: Judit Novak <judit.novak@gmail.com>

ArrayOf metadata existed before the LuaCATS type documentation refactor and is
useful to typed clients consuming api_info().

Keep Tuple metadata normalized to Array, since tuple element types can be mixed,
but preserve ArrayOf(...) for exported API metadata.

Fixes #38734

AI-assisted: Codex

Allow changing return-type from `void => non-void`.

- Render Lua types in api.txt.

- Added `DictAs(name)` API type which acts the same as `Dict` (no parens)
  when generating the dispatchers, but acts the same as `Dict(name)`
  when generating docs.

- Added `Tuple(...)` API type which is the treated the as `Array` for
  generating the dispatchers, but is used to document richer types.

- Added `Enum(...)` API type to better document enums

- Improve typing of some API functions.

- Improve c_grammar to properly parse API types and replace string pattern
  logic in the parsers.

- Removed all the hardcoded type overrides in gen_eval_files.lua