change assert.ok to narrow the type

[ryanatkn]

This changes the return type of assert.ok from void to asserts actual, making type narrowing work:

type A = {ok: true, data: any} | {ok: false};
const a = {ok: true, data: 1} as A;
assert.ok(a.ok);
a.data; // type error! without this change, type does not narrow

This is the same as assert.ok in @types/node:

function ok(value: any, message?: string | Error): asserts value;

This is more complicated for assert.not.ok. The syntax asserts !val does not work, you have to say asserts val is $type. So, does a falsy union type like asserts actual is false | '' | null | undefined | 0 | -0 | typeof NaN work correctly for all cases? It does for my code, but I don't know if it's correct - I think it is? As a workaround, you can negate the condition and use ok to get that behavior with this PR. I'll follow up on this and some other types in an issue soon. (sorry this may be straightforward, but I believe I couldn't get this to work in a previous TS version, but it works now for the cases I have)

AFAIK this change is only a win - because it only narrows the type, I don't think it can cause errors. The other changes I want to make may break existing typechecks.

[lukeed]

Thanks! Could probably do with some tricks for is, equal, and instance too but it's probably not worth the effort. This one for ok is a safe addition