Rename 'MaybePromise' to 'PromiseOrValue'

[IvanGoncharov]

Suggested by @martijnwalraven
MaybePromise name confuses users into thinking: execute/graphql returns either Promise or null/undefined.
Also in our codebase, we frequently use [Mm]aybe* to represent optional things,
e.g.:

graphql-js/src/language/visitor.js

Line 353 in 8c96dc8

function isNode(maybeNode): boolean %checks {

graphql-js/src/language/printer.js

Line 244 in 8c96dc8

function join(maybeArray, separator) {

[IvanGoncharov]

@Cito What do you think? Maybe you have a better naming proposal?

[IvanGoncharov]

Note: not breaking change, since it's part of jsutils:
https://github.com/graphql/graphql-js/blob/master/src/jsutils/README.md

These functions are not part of the module interface and are subject to change.

[Cito]

The name MaybePromise is also used by others, but @martijnwalraven has a point when he says Maybe already means "or null/undefined" in GraphQL.js elsewhere, and Flow also has the concept of Maybe Types for optional values. PromiseOrValue sounds good to me. I found that name also in the typescript definition for Promise.all(). So renaming maybe (pun intended) makes sense.

[mjmahone]

This seems like a solid clarity win to me, so I'm all for it.

[IvanGoncharov]

Maybe already means "or null/undefined" in GraphQL.js elsewhere, and Flow also has the concept of Maybe Types for optional values. PromiseOrValue sounds good to me. I found that name also in the typescript definition for Promise.all().

Thanks for research 👍
I think naming is important for graphql-js as the reference implementation since readability is one of the top priorities.