[libpromise.js] Add explicit addPromise helper function

[sbc100]

In addition to making things more explicitly, this saves on codesize when more than one promise function is included (see dylink_all codesize changes).

[tlively]

"explicitly" => "explicit" in the description, but lgtm.

How do you decide when adding a tiny helper like this is worth it for code size benefits? It seems like you would structure lots of code differently if you were really trying to code golf it.

[sbc100]

"explicitly" => "explicit" in the description, but lgtm.

How do you decide when adding a tiny helper like this is worth it for code size benefits? It seems like you would structure lots of code differently if you were really trying to code golf it.

TBH, what inspired me to make this change was readability. I was working on #26947 and I didn't at first see how promises were added to the mapping other than makePromise .. which creates and extra promise, which I didn't want to do.

Having both makePromise and addPromise side by side makes it clear there are two ways to do things rather than distributing the second method around the codebase.

[sbc100]

Also this just seemed better than repeating the .. {promise: ...} pattern everywhere