feat(json): vim.json.encode() sort_keys #35574

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.

runtime/lua/vim/_meta/json.lua

@@ -56,5 +56,7 @@ function vim.json.decode(str, opts) end
 ---                                   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.
+---                                 - sort_keys: (boolean) (default false) Sort object
+---                                   keys in alphabetical order.
 ---@return string
 function vim.json.encode(obj, opts) end