Fix FluentDateTime and FluentNumber primitive conversions

[julienw]

This fixes a problem when using a custom function taking a FluentDateTime as a parameter, like we're doing in the profiler code.

We're relying on being able to convert a FluentDateTime value to a number:
https://github.com/firefox-devtools/profiler/blob/f699b44fcf763bdfb6ebf36c32d668e4d296f5d6/src/utils/l10n-ftl-functions.js#L87-L91

export const SHORTDATE: FluentFunction = (args, _named) => {
  const date = args[0];
  const nowTimestamp = Date.now();


  const timeDifference = nowTimestamp - +date;

used in:
https://github.com/firefox-devtools/profiler/blob/9c8fb5561537894aee2fe8a265d2c127e83ed0e4/locales/en-US/app.ftl#L679-L683

# The function SHORTDATE is specific to the profiler. It changes the rendering
# depending on the proximity of the date from the current date.
# Variables:
#   $date (Date) - The date to display in a shorter way
NumberFormat--short-date = { SHORTDATE($date) }

The good thing is that the code was already present as a toNumber function. It's not clear to me why the original code didn't use valueOf, but doing it in this patch fixes the problem for me.

I actuallty tested locally with our profiler code, that it fixes the issue.

Hope this solution works for you :-)

[eemeli]

As discussed on Matrix, I took over this PR from @julienw as the proper fix here required a bit more finessing. Effectively, as #636 changed the FluentDateTime value and valueOf() result to not always be a number, casting a FluentDateTime to a number now also tries to call toString() (due to primitive conversion rules), which fails because it's expecting to get a non-empty scope argument.

So to make all of this work and not create further breaking changes, the right thing to do is to:

1. use Symbol.toPrimitive to more explicitly control the conversion, and
2. allow toString() to be called without an argument.