feat(a380x): true/mag button

[tracernz]

Fixes #[issue_no]

Summary of Changes

Clear out the git stash... Adds the TRUE/MAG button on the FCU.

Screenshots (if necessary)

References

Additional context

Discord username (if different from GitHub):

Testing instructions

• Spawn in a region at lower latitude (so the magnetic database is available). Press the true/mag button and ensure the PFD and ND switch to true ref mode. Press it again and make sure they switch back.

How to download the PR for QA

Every new commit to this PR will cause new A32NX and A380X artifacts to be created, built, and uploaded.

1. Make sure you are signed in to GitHub
2. Click on the Checks tab on the PR
3. On the left side, find and click on the PR Build tab
4. Click on either flybywire-aircraft-a320-neo or flybywire-aircraft-a380-842 download link at the bottom of the page