More accurate unquoted command args. (#132)

It turns out that external scanner is not necessary to fix most cases in
#128 .

One existing test case modified as the original entry is actually parsed
as string in nu 0.99.1:

<img width="714" alt="image"
src="https://github.com/user-attachments/assets/2a90afa0-d6fc-4512-8077-34a8fda98205">

More corner cases added to test.