Merge 'Add support for json_object function' from Jorge Hermo

Relates to #127.  This PR is still in draft and I have a few left things
to do (tests, improve implementation), just opening it so anyone can
track this work meanwhile.

Closes #664

testing/json.test

@@ -506,3 +506,41 @@ do_execsql_test json_error_position_null {
 do_execsql_test json_error_position_complex {
   SELECT json_error_position('{a:null,{"h":[1,[1,2,3]],"j":"abc"}:true}');
 } {{9}}
+
+do_execsql_test json_object_simple {
+  SELECT json_object('key', 'value');
+} {{{"key":"value"}}}
+
+do_execsql_test json_object_nested {
+  SELECT json_object('grandparent',json_object('parent', json_object('child', 'value')));
+} {{{"grandparent":{"parent":{"child":"value"}}}}}
+
+do_execsql_test json_object_quoted_json {
+  SELECT json_object('parent', '{"child":"value"}');
+} {{{"parent":"{\"child\":\"value\"}"}}}
+
+do_execsql_test json_object_unquoted_json {
+  SELECT json_object('parent', json('{"child":"value"}'));
+} {{{"parent":{"child":"value"}}}}
+
+do_execsql_test json_object_multiple_values {
+    SELECT json_object('text', 'value', 'json', json_object('key', 'value'), 'int', 1, 'float', 1.5, 'null', null);
+} {{{"text":"value","json":{"key":"value"},"int":1,"float":1.5,"null":null}}}
+
+do_execsql_test json_object_empty {
+  SELECT json_object();
+} {{{}}}
+
+do_execsql_test json_object_json_array {
+    SELECT json_object('ex',json('[52,3]'));
+} {{{"ex":[52,3]}}}
+
+do_execsql_test json_from_json_object {
+    SELECT json(json_object('key','value'));
+} {{{"key":"value"}}}
+
+# FIXME: this behaviour differs from sqlite. Although, sqlite docs states
+# that this could change in a "future enhancement" (https://www.sqlite.org/json1.html#jobj)
+#do_execsql_test json_object_duplicated_keys {
+#    SELECT json_object('key', 'value', 'key', 'value2');
+#} {{{"key":"value2"}}}