Problem:
- Not obvious which _meta/ are generated and which should be edited
  manually.
- The require guard (`error('Cannot require a meta file')`) is not
  consistently present in all meta files.

Solution:
- Update headers.
- Add require() guard to all meta files.
- Rename generated meta files with `.gen.lua`.

Problem:
`vim.json.decode()` could not parse JSONC (JSON with Comments)
extension, which is commonly used in configuration files.

Solution:
Introduce an `skip_comments` option, which is disabled by default. When
enabled, allows JavaScript-style comments within JSON data.

Close #35926
Close #35818

Co-authored-by: skewb1k <skewb1kunix@gmail.com>
Co-authored-by: glepnir <glephunter@gmail.com>

Problem: There is no way to ensure a stable key order when encoding a JSON string,
which can be useful for comparisons and producing cleaner diffs.

Solution: Introduce a `sort_keys` option for `vim.json.encode()`,which
is disabled by default. When enabled, object keys are sorted in
alphabetical order.

Problem:
There is no straightforward way to pretty-print objects as JSON.
The existing `vim.inspect` outputs LON.

Solution:
Introduce an `indent` option for `vim.json.encode()` which enables
human-readable output with configurable indentation.

Adapts PR to upstream: openresty/lua-cjson#114

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.)