[cli] Refactor legacy Now client to TypeScript

[TooTallNate]

Refactors one of the last remaining big JS files to TypeScript. I'd prefer to eliminate this file completely, but it's still very tightly coupled to the deployment process, so figured it was easier to just bite the bullet and refactor it to TS. This is necessary because the Jest unit tests don't like the .js files that use import (I'm sure there's a way to configure babel within Jest to support es6 module syntax, but since we want the code to be TS anyways, this is more killing two birds with one stone).

[codecov]

Codecov Report

Merging #6650 (40dc088) into main (273718e) will not change coverage.
The diff coverage is 25.00%.

❗ Current head 40dc088 differs from pull request most recent head 6f66cba. Consider uploading reports for the commit 6f66cba to get more accurate results

@@           Coverage Diff           @@
##             main    #6650   +/-   ##
=======================================
  Coverage   54.98%   54.98%           
=======================================
  Files          85       85           
  Lines        3592     3592           
  Branches      841      841           
=======================================
  Hits         1975     1975           
  Misses       1611     1611           
  Partials        6        6           

Impacted Files	Coverage Δ
packages/cli/src/util/config/files.ts	30.43% <0.00%> (ø)
packages/cli/src/util/client.ts	70.93% <100.00%> (ø)

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