feat: Allow a local configuration file for custom compiles (#879)

## Summary

* platformio.ini is a repository based config for platformio and cannot
be modified without constant nagging of git to include it into you
commits
* PlatformIO allows you to split your configuration into multiple files
using the extra_configs option in the [platformio] block. This
effectively merges other .ini files into your main one. This will be
silently ignored if such a file does not exist

## Additional Context

* Modifiy platformio.ini and add a .gitignore entry to ignore your local
config
* eg my own ``platformio.local.ini``:
```
[env:custom]
extends = base
build_flags =
  ${base.build_flags}
  -DCROSSPOINT_VERSION=\"${crosspoint.version}-custom\"
  ; inclusion of additional fonts is disabled in custom builds to save space
  -DOMIT_FONTS
```
---

### AI Usage

While CrossPoint doesn't have restrictions on AI tools in contributing,
please be transparent about their usage as it
helps set the right context for reviewers.

Did you use AI tools to help write this code? _NO_

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

platformio.ini

@@ -1,5 +1,6 @@
 [platformio]
 default_envs = default
+extra_configs = platformio.local.ini
 
 [crosspoint]
 version = 1.0.0