Add mapNotNullish to collections

[LionC]

This adds mapNotNullish<T, O>(array: Array<T>, transformer: (el: T) => O): Array<NonNullable<O>> to collections. This method helps in common cases like extracting an optional property from objects or mapping and filtering an array at the same time:

Example 1:

const processes = [
    { name: "Building", problem: null },
    { name: "Testing", problem: "Could not load test file" },
    { name: "Benchmarking", problem: "OutOfMemory" },
    { name: "Deploying", problem: undefined },
]

const problems = mapNotNullish(processes, it => it.problem)
// [ "Could not load test file", "OutOfMemory" ]

Example 2:

const numbers = [ 1, 2, 3, 4 ]

const evensSquared = mapNotNullish(numbers, it => it % 2 === 0 ? it*it : null)
// [4, 16]

mapNotNullish(someArray, someFunction) has the same result as

someArray
    .map(someFunction)
    .filter((it) => it !== undefined && it !== null)

but is more efficient regarding runtime and memory (only one iteration, no intermediate array etc), states the intent shorter and more specifically and gets the types right (which filter does not without explicit type guards).

Like a lot of the collections functions, mapNotNullish() is heavily inspired by kotlins .mapNotNull().

[kt3k]

@LionC LGTM.

As explained in the description, this is optimized version of .map().filter() idiom. I think this has decent use cases, especially when the source array is large.

[kt3k]

We prefer ts tag, but let's address this later.

[caspervonb]

Won't be typechecked, but I'll address that upstream in cli.

[LionC]

Oh sorry - I keep forgetting that. Should I post a separate PR?