feat(json): pretty-format (indent) with vim.json.encode() #35424

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

runtime/lua/vim/_meta/json.lua

@@ -38,5 +38,7 @@ function vim.json.decode(str, opts) end
 ---@param opts? table<string,any> Options table with keys:
 ---                                 - escape_slash: (boolean) (default false) Escape slash
 ---                                   characters "/" in string values.
+---                                 - indent: (string) (default "") String used for indentation at each nesting level.
+---                                   If non-empty enables newlines and a space after colons.
 ---@return string
 function vim.json.encode(obj, opts) end