Problem:
vim.json.encode escapes every slash in string values (for example in
file paths), and is not optional. Use-case is for preventing HTML
injections (eg. injecting `</script>` closing tag); in the context of
Nvim this is rarely useful.

Solution:
- Add a `escape_slash` flag to `vim.json.encode`.
- Defaults to `false`. (This is a "breaking" change, but more like
  a bug fix.)

Note: Upstream doesn't have this. It's an Nvim addition.

Specifically, functions that are run in the context of the test runner
are put in module `test/testutil.lua` while the functions that are run
in the context of the test session are put in
`test/functional/testnvim.lua`.

Closes https://github.com/neovim/neovim/issues/27004.

Work on https://github.com/neovim/neovim/issues/27004.

Problem:
Empty string is a valid JSON key, but json_decode() treats an object
with empty key as ":help msgpack-special-dict". #20757

    :echo json_decode('{"": "1"}')
    {'_TYPE': [], '_VAL': [['', '1']]}

Note: vim returns `{'': '1'}`.

Solution:
Allow empty string as an object key.

Note that we still (currently) disallow empty keys in object_to_vim() (since 7c01d5ff9286d262097484c680e3a4eab49e2911):
https://github.com/neovim/neovim/blob/f64e4b43e1191ff30d902730f752875aa55682ce/src/nvim/api/private/converter.c#L333-L334

Fix #20757

Co-authored-by: Justin M. Keyes <justinkz@gmail.com>

Problem:
- `vim.json` exposes various global options which:
  - affect all Nvim Lua plugins (especially the LSP client)
  - are undocumented and untested
  - can cause confusing problems such as: https://github.com/codota/tabnine-nvim/commit/cc76ae3abe2f129d44b5a8edee2529e0ee0dcf69
- `vim.json` exposes redundant mechanisms:
  - `vim.json.null` is redundant with `vim.NIL`.
  - `array_mt` is redundant because Nvim uses a metatable
    (`vim.empty_dict()`) for empty dict instead, which `vim.json` is
    configured to use by default (see `as_empty_dict`).
    Example:
    ```
    :lua vim.print(vim.json.decode('{"bar":[],"foo":{}}'))
    --> { bar = {},  foo = vim.empty_dict() }
    ```
    Thus we don't need to also decorate empty arrays with `array_mt`.

Solution:
Remove the functions from the public vim.json interface.
Comment-out the implementation code to minimize drift from upstream.

TODO:
- Expose the options as arguments to `vim.json.new()`