Fix two problems that made it difficult to convert between Message
and MessageValue, or to write code that could accept both of them,
as exemplified by the StatusValue class:
* Implement a common interface
* Use the same internal format for message parameters
While these changes should be compatible with most of existing code,
where the authors were courteous enough to simply call methods such
as Message::numParam() and not look inside the values they return,
it is potentially a breaking change for anything that depended on
the formatted params being arrays or accessed their keys.
Example patches: https://gerrit.wikimedia.org/r/q/topic:message-param
Notable changes:
* Message and MessageValue now both implement MessageSpecifier
(only Message implemented it before).
* Message::numParam() and other static methods for encoding params
now return MessageParam objects, instead of special arrays.
Use these MessageParam objects internally in Message.
* Narrow down the return type of MessageSpecifier::getParams() (it
was just `array`, allowing any type in the array). Narrow down the
types for Message::params() and MessageValue::params() to match.
* Deprecate MediaWiki\Message\Converter. As a replacement add
MessageValue::newFromSpecifier(), which is analogous to
Message::newFromSpecifier(), but without weird legacy edge cases.
* Make StatusValue::getMessages() return MessageValues. Remove code
that converted between Message and MessageValue, no longer needed.
* Update many type declarations and comments to use MessageSpecifier
instead of MessageValue, as well as a couple of tests that depended
on implementation details.
Bug: T358779
Change-Id: I625a48a6ecd3fad5c2ed76b23343a0fef91e1b83
9d56257d8c