Add :percent

[eemeli]

Closes #956

As discussed on the preceding weekly calls, it looks like the least worst best option for us is to add a dedicated :percent function, to match what we're doing splitting :currency and :unit from :number.

This PR adds that function, initially as Draft. It also updates the related design doc to match this choice, and updates the test suite.

To make it into LDML 48, this PR will need to land by 2025-08-18 at latest.

[aphillips]

One comment on the design doc, which is optional.

Thank you for wrapping up percent!!

[eemeli]

@catamorphism Thank you for catching the loose use of "resolved value". I've now replaced the places where that's used with "numeric value", which is what we use in describing :offset.

@macchiati I've moved your comment to be its own new issue #1095, as it goes beyond the scope of this PR, and we should discuss and address it separately.

[catamorphism]

Sorry to nitpick, but this is still a little confusing; the resolved value returned by the function can't be altered, because the function handler (logically) creates a new resolved value to return. The input resolved value is immutable.

Perhaps something like: "The numerical value of the operand is the same as the numerical operand of the resolved value of the expression: it is not multiplied by 100."

[eemeli]

@catamorphism I revised the text to match your suggestion; PTAL?