Add support for roaming settings.json or storing it elsewhere

Note: 📌 Pinned comment: https://github.com/microsoft/terminal/issues/2933#issuecomment-536652883

Description of the new feature/enhancement

I have three different computers that I use for work. I keep my PowerShell profile in a GitHub repository and dot source it in my local PowerShell profile. That way I will only need to do a git pull on my profile repository to get all changes propagated on each computer.

It would be great if I could do something similar with my Windows Terminal settings by simply stating in my local settings.json file that the "real" settings could be found somewhere else.

(It could even be that the loaded profile acts like a base for the settings on the computer so things could be overridden, but that is a completely different issue that remains to be opened)

I'm aware that I might have overlooked something here, but would love to start a discussion about this since I suspect that I'm not the only one with this "problem".

Proposed technical implementation details (optional)

{
    "$schema": "https://aka.ms/terminal-profiles-schema",
    "globals": {
        "loadProfileFrom": "C:/Source/git/profiles/terminal_profile.json"
    }

Start with the pinned comment and the settings.json example in this issue, then review the profiles schema URL to understand the proposed loadProfileFrom setting. The issue does not name source files or tests, so locating the settings-loading entry point and defining override behavior will require project familiarity. Done would mean Windows Terminal can load settings from an alternate location as described.