test: clean up typecast through type assertions

[arthurschreiber]

This removes the @types/chai package and instead places a copy of the type definitions to types/chai.d.ts.

I made a few changes to the type definitions for chai and opened a pull request over at DefinitelyTyped/DefinitelyTyped#60464. Unfortunately, the changes haven't been merged yet, and I didn't have time to fix up the PR.

To clean up the tests in tedious, I decided that it'll be easier to vendor the type definitions for chai for the time being, and then re-open that pull request at some later time.

One of the changes I made to the chai definitions is to change assert.instanceOf to perform a type assertion. This helps TypeScript's understanding of what types a variable can have after the call to the assertion method. A similar change was also done to assert.exists.

[codecov]

Codecov Report

Merging #1484 (c7e3e36) into master (ece396a) will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1484      +/-   ##
==========================================
- Coverage   80.48%   80.45%   -0.03%     
==========================================
  Files          91       91              
  Lines        4657     4657              
  Branches      856      856              
==========================================
- Hits         3748     3747       -1     
- Misses        633      634       +1     
  Partials      276      276              

Impacted Files	Coverage Δ
src/connection.ts	65.14% <0.00%> (-0.10%)	⬇️

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