## Summary of the Pull Request
This looks like a big diff, but there's a bunch of existing code that
just got moved around, and there's a cool new Utils template.
The tests all pass, and this passed manual validation. I tried weird
things like "making a profile named `{ }`"
(w/ enough spaces to look like a guid), and yeah it doesn't let you
specify that one as a name, but _why would you do that?!_
Okay, this pull request abstracts the conversion of a profile name into
an optional profile guid out of the "New Terminal Tab Args" handler and
into a common space for all of CascadiaSettings to use.
It also cleans up the conversion of indices and names into optional
GUIDs and turns _those_ into further helpers.
It also introduces a cool new template for running value_or multiple
times on a chain of optionals. CoalesceOptionals is a "choose first,
with fallback" for N>1 optionals.
On top of all this, I've built support for an "unparsed default GUID":
we load the user's defaultProfile as a string, and as part of settings
validation we unpack that string using the helpers outlined above.
## References
Couples well with #5690.
## PR Checklist
* [x] Incidentally fixes#2876
* [x] Core Contributor
* [x] Tests added/passed
* [x] Requires documentation to be updated (done)
* [x] I've discussed this with core contributors already
## Validation Steps Performed
Added additional test collateral to make sure that this works.
9ed776bf3e