Fix/update uppercase custom function example per latest messageformat@next

[lionel-rowe]

• Param types fixed (first arg is context: MessageFunctionContext, not locales: string[])
• Return type fixed — only including toString doesn't work with format or formatToParts
• Use toLocaleUpperCase with the locales available in the context, rather than locale-less toUpperCase

It seems like building on top of the built-in DefaultFunctions.string is the simplest way to implement a custom string -> string function that correctly implements the contract expected by format and formatToParts. The returned object for input "{messageformat :uppercase}" looks roughly like this:

{
  type: "string",
  source: "|messageformat|",
  dir: "auto",
  selectKey: (keys) => (keys.has(selStr) ? selStr : null),
  toParts: () => [{ type: "string", source: "|messageformat|", locale: "en", value: "MESSAGEFORMAT" }],
  toString: () => "MESSAGEFORMAT",
  valueOf: () => "MESSAGEFORMAT"
}