Remove dependency on chai

[ghost]

Removes the over 22000 line chai package and replaces it with under 50 lines of assertion functions.

(Background: Builds were failing because the latest @types/chai no longer creates a global variable for types. Normally this is a good thing but without #14844 we can no longer refer to the type in a global context.)

[sandersn]

Looks good, but I notice that the build still failed on this PR. Any idea why?

[ghost]

The tests succeed when run directly. At some point in the tests there's a global state change such that [] is no longer deeply equal to []. I'm looking into it.
(General global-state-in-tests tracking issue: #10683)

[mhegazy]

Should we update the @types/chai dependency to an older version for now?

[mhegazy]

At some point in the tests there's a global state change such that [] is no longer deeply equal to []

this is concerning.. I see you removed use of deepEqual from some tests, but what is the root cause?

[ghost]

I can use the previous commit as a repro but it will take a while to find out what went wrong. Can we merge this in the meantime?

[ghost]

Fix for global state is at #20471

[weswigham]

I have no idea if we have any tests which could trigger this (probably not), but NaN === NaN is false, but I'd generally say one would expect { x: NaN } to deep equal { x: NaN }; ergo there should probably be a clause if (a !== a && b !== b) return true;. Probably a pointless remark unless we start testing more NaNy edgecases, though.

[weswigham]

It's also worth noting that this change is going to give worse error output when debugging, since you'll no longer get pretty object diffs from any of the error messages from our custom isDeepEqual like chai would give.