[LIQ] Add nulls first / nulls last to order by

Extend the `order by` clause with optional `nulls first` and `nulls
last` modifiers following standard SQL semantics. Each sort key can
independently specify where `nil` values should appear in the result. By
default, ascending sorts place nulls last and descending sorts place
nulls first. An explicit nulls first or nulls last overrides this
default. The `asc` keyword is also now accepted as an explicit
alternative to the default ascending direction. Documentation and tests
were updated accordingly.

Signed-off-by: Matouš Jan Fialka <mjf@mjf.cz>

client/space_lua/ast.ts

@@ -322,6 +322,7 @@ export type LuaOrderBy = {
   type: "Order";
   expression: LuaExpression;
   direction: "asc" | "desc";
+  nulls?: "first" | "last";
 } & ASTContext;
 
 export type LuaSelectClause = {