Add 'show_size' option to target.json file to print build size info.

[microbit-carlos]

If we decide to merge this CODAL build system feature, this is how to activate it:

• Add new target option to print/show the build size. microbit-v2-samples#81

Discussions are probably better over that PR.

[github-actions]

Build diff

Base commit: ffe0265a9530c337c2520a407e1d7b11aa36a78e
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/6121824064

     VM SIZE    
 -------------- 
  [ = ]       0    TOTAL

[finneyj]

thanks @microbit-carlos!

[github-actions]

Build diff

Base commit: 7197022905525956cc740ea4fc924ae03f3ed439
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/13413159517

     VM SIZE    
 -------------- 
  [ = ]       0    TOTAL

[microbit-carlos]

Thanks Joe!

As lancaster-university/microbit-v2-samples#81 has been merged already I've rebased this branch and it's ready to be merged 👍