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/doc/lua.txt

@@ -3391,6 +3391,8 @@ vim.json.encode({obj}, {opts})                             *vim.json.encode()*
                 • 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`)

runtime/doc/news.txt

@@ -266,6 +266,7 @@ LUA
 • |vim.list.bisect()| for binary search.
 • Experimental `vim.pos` and `vim.range` for Position/Range abstraction.
 • |vim.json.encode()| has an `indent` option for pretty-formatting.
+• |vim.json.encode()| has an `sort_keys` option.
 
 OPTIONS