CI: Add workflow to calculate build size diff before & after PR/commit.

[microbit-carlos]

This PR uses Google's Bloaty McBloatface to analyse the MICROBIT elf file for memory consumption.

It also builds the PR base commit, or the commit parent, to do a diff and see the additional memory consumed or saved.

In the case of being a PR from a branch in this repository (not forks) it will post the diff as a comment as well.

To show an example diff in this PR (instead of an output just showing "no change in size"), the initial commit will contain a bit of code in MicroBit.cpp to create a 4000 byte array and a loop to write some data into it. So, the first bot comment will show the PR as increasing the .bss and .text sections.
I will then force push an update removing this code and the bot will post another comment showing a zero size increase.

Both bloaty analysis outputs are added to to the Action work summary, so even PRs from forks will have this info (even if they don't get a PR comment):

[github-actions]

Build diff

Base commit: 6c1433e877e570f9958c1edab77062d4ed9220fd
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/4074154437

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  [ = ]       0   +44% +3.91Ki    .bss
  +0.0%     +16  +0.0%     +16    .text
  +0.0%     +32  [ = ]       0    .debug_abbrev
  +0.0%    +103  [ = ]       0    .debug_info
  +0.0%     +48  [ = ]       0    .debug_line
  +0.0%     +19  [ = ]       0    .debug_loc
  +0.0%     +24  [ = ]       0    .debug_ranges
  +0.0%     +20  [ = ]       0    .debug_str
  +0.0%     +20  [ = ]       0    .strtab
  +0.0%     +32  [ = ]       0    .symtab
  -0.0%     -18  [ = ]       0    [Unmapped]
  [ = ]       0  -3.6% -3.91Ki    .heap
  +0.0%    +296  +0.0%     +16    TOTAL

[microbit-carlos]

Okay, so now I'll force push without the test C++ code and the bot should add another comment after this, indicating the build size for this PR hasn't changed vs the base commit.

https://github.com/lancaster-university/codal-microbit-v2/compare/971d6c59b66015612f17f3f7a69c1f217eed3325..7843574ae2a196029527eb79cfb004d23e0be385

[github-actions]

Build diff

Base commit: fd3bde3c71e381ce2f6fd732433bdba5c528782e
Action run: https://github.com/lancaster-university/codal-microbit-v2/actions/runs/4167952013

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

[microbit-carlos]

I've tweaked a bit the order of the workflow steps and how the codal.json file is modified and this PR is now ready to be merged 🎉