lua: add utils.format_json() function

Requested. Why not.

DOCS/man/lua.rst

@@ -639,6 +639,14 @@ strictly part of the guaranteed API.
     trailing text is returned as 3rd return value. (The 3rd return value is
     always there, but with ``trail`` set, no error is raised.)
 
+``utils.format_json(v)``
+    Format the given Lua table (or value) as a JSON string and return it. On
+    error, returns ``nil, error``. (Errors usually only happen on value types
+    incompatible with JSON.)
+
+    The argument value uses similar conventions as ``mp.set_property_native()``
+    to distinguish empty objects and arrays.
+
 ``utils.to_string(v)``
     Turn the given value into a string. Formats tables and their contents. This
     doesn't do anything special; it is only needed because Lua is terrible.

player/lua.c

@@ -1255,6 +1255,23 @@ static int script_parse_json(lua_State *L)
     return 3;
 }
 
+static int script_format_json(lua_State *L)
+{
+    void *tmp = mp_lua_PITA(L);
+    struct mpv_node node;
+    makenode(tmp, &node, L, 1);
+    char *dst = talloc_strdup(tmp, "");
+    if (json_write(&dst, &node) >= 0) {
+        lua_pushstring(L, dst);
+        lua_pushnil(L);
+    } else {
+        lua_pushnil(L);
+        lua_pushstring(L, "error");
+    }
+    talloc_free_children(tmp);
+    return 2;
+}
+
 #define FN_ENTRY(name) {#name, script_ ## name}
 struct fn_entry {
     const char *name;
@@ -1303,6 +1320,7 @@ static const struct fn_entry utils_fns[] = {
     FN_ENTRY(join_path),
     FN_ENTRY(subprocess),
     FN_ENTRY(parse_json),
+    FN_ENTRY(format_json),
     {0}
 };