Allow numeric filenames as output (`1.gif`)

Using `Output 1.gif` in a .tape file breaks with a friendly error.

```
File: test.tape
1 │ Output 1.gif
^^^^^^ Expected file path after output

1 │ Output 1.gif
^^ Invalid command: 1.

1 │ Output 1.gif
^^^ Invalid command: gif

Error: parse error
```

Perhaps it could be allowed?
(Of course, adding an alpha character to the path works.)

Reproduce the parse error with the test.tape example containing `Output 1.gif`, then find the parser entry point that handles Output commands. The work is complete when a numeric filename such as `1.gif` is accepted and the existing parse error no longer occurs.

Written by the indexing model from the issue text.