Clarify `standalone` vs. `open` vs. `close` function/expression

[aphillips]

See: https://github.com/unicode-org/message-format-wg/pull/419/files#r1262730738

In adding definitions for open and close expressions (and their friend standalone), we found that we are underspecifying them. This is a placeholder to solve that.

[stasm]

For completeness and ease of viewing, here's @aphillips's comment from the review thread linked above:

In particular, the current state of our spec is that these are all the same and refer to a discrete function open:

let $foo = {$bar :open}
let $foo = {$bar +open}
let $foo = {$bar -open}

Obviously, though, +open and -open need to have a different syntactical "meaning" if the open and close expressions are to do different things (such as write open vs. close tags). The interaction with the function registry is interesting here.

[stasm]

There are two requirements that I'd like to suggest, in line with @aphillips's comment:

1. It should be possible to make a placeholder "open" or "close" even when it doesn't mention a function name at all.

   • Corollary: standalone/open/close are a property of the placeholder itself rather than the function's.

2. It should be possible to define a single function in the registry (e.g. :html) and then specify its various possible signatures for formatting or matching. The formatting signatures should also define whether they're for standalone, open, or close uses.

   <function name="html">
       <formatSignature position="open">
            <!-- ... -->
       </formatSignature>
   
       <formatSignature position="close">
            <!-- ... -->
       </formatSignature>
   </function>
   

   • Corollary: +html and -html (in today's syntax) are two signatures of the same function.

These two requirements combined lead me to suspect that we should move the open/close syntax into the placeholder rather than bind it to the function name.

[eemeli]

I have a few thoughts on this:

1. The exact shape of the XML doesn't really matter to me, i.e. whether signatures are nested under a single <function> or all next to each other is not important.
2. +html and -html need to have different options baskets.
3. It would be useful to be able to use a single definition for multiple functions. For example, +bold and +italic (or +html.b and +html.i) would probably accept the same options (if any!), along with many others. It'd be nice not to need to repeat the same thing multiple times.
4. I would really like not needing to revisit expression syntax again.

[markusicu]

This issue looks like it has been superseded by https://github.com/unicode-org/message-format-wg/blob/main/exploration/open-close-placeholders.md and #537