feat(oiiotool): enhance expressions with string formatting operator

It would be handy for the command line expression syntax (things like `{2 * TOP.width}`) had a fully general way to do string formatting.

This may be kind of involved, but I think it's worth doing.

We'll use this issue to track progress and flesh out a spec for it.

As a motivating example, see #5207, where we posit the utility of something like

--offset "{format('{:+}{:+}',X,Y}"

There's a bit of ugliness coming from the fact that we use `{}` in oiiotool for indicating expressions, but also `{}` inside the format specifications, and I'm not sure if it will end up needing special quoting. But the advantage of going for a fully general formatting utility that can be used in expression evaluation means that it not only solves your problem, but likely also unlocks a whole bunch of interesting additional usages.

Start by reviewing oiiotool's command-line expression syntax and the motivating example in issue #5207. Define how a general string-formatting operator interacts with the existing expression braces and quoting rules; done means the specification is settled and the requested formatting use case works without ambiguity.

Written by the indexing model from the issue text.