feat(json): sort_keys option to vim.json.encode()

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.

Adapts PR to upstream: openresty/lua-cjson#115.

Author: skewb1k

runtime/doc/lua.txt

@@ -3375,6 +3375,8 @@ vim.json.encode({obj}, {opts})                             *vim.json.encode()*
       • {opts}  (`table<string,any>?`) Options table with keys:
                 • escape_slash: (boolean) (default false) Escape slash
                   characters "/" in string values.
+                • sort_keys: (boolean) (default false) Sort object keys in
+                  alphabetical order.
 
     Return: ~
         (`string`)

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