Edit BASIC heuristic to avoid false positives + new sample

[DecimalTurn]

There is currently at least 2 occurences of false positives with the BASIC heuristic ('^\s*\d+') introduced in #5166:

1. If a VBA file contains line numbers, it will automatically be identified as BASIC. It is not that common for VBA code to have line numbers, but some people like to use them to improve error reporting.
   eg. https://github.com/Sven-Bo/Integrate-ChatGPT-in-Excel-using-VBA/blob/master/mChatGPT.bas#L34

2. In VBA (or VB6), an underscore can be used as a line continuation character which means that lines can be split and there is the possibility that a number ends up being the first thing on the second line thus matching with the heuristic.
   eg. https://github.com/dragokas/hijackthis/blob/7560e67b5e0d14f53ea25bbfb88587cbfadca1c8/src/modPermissions.bas#L905

The good thing is that both of these issues can't happen at the start of the file because you can't have line numbering outside of a procedure in VBA and it can't be a line continuation if it's the start of the file (obviously).

Hence, I'm proposing to add \A at the start of the heuristic (which gives ^\A\s*\d+) and I also added a sample so that we have two samples for BASIC with the .bas extension.

Description

Checklist:

• I am fixing a misclassified language
  • I have included a new sample for the misclassified language:
    • Sample source(s):
      • GPIOGW.BAS
    • Sample license(s): GPL v3
  • I have included a change to the heuristics to distinguish my language from others using the same extension.

[lildude]

Good explanation and makes sense to me. Thanks.