feat(dynamic): add dynamic wrapper function

[onelson]

Originally planned as a part of #5143, this diff adds a new builtin to the experimental/dynamic package which wraps an existing value in a Dynamic.

This implementation is weak, probably incomplete, and hard to test without additional work since part of the contract of our "restrictive" dynamic type is you can't really do much with it until you cast it to some other value.
The buried lede is: we haven't implemented the dynamic support for the standard cast fns yet.

At this early stage the best I can offer in terms of testing is this short REPL session:

❯ go run ./cmd/flux
> import dyn "experimental/dynamic"
> dyn.dynamic(v: 123)
<unknown value>
> dyn.dynamic(v: 123) == dyn.dynamic(v: 123)
Error: unsupported binary expression dynamic == dynamic
>

I went ahead and added the == case to the test file just so we can get the addition of the file in the repo, and to have some sort of test in place. There's not a whole lot we can do right now other than prove the function is something you can import.

Checklist

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[wolffcm]

Looks great.

[onelson]

Apologies to @sanderson. I'd like to offer some practical examples in the docstring, but nothing will actually fly atm. I'll aim to loop back as we flesh out the functionality.

[sanderson]

Just some minor suggestions. Looks really good.

[sanderson]

👌🏻 Looks great.