Summary

This is a PR that should be merged into #1024 and incrementally addresses #842.

This PR adds test-code-specific rules:

• Turns off the prefer-arrow-callback rule - but only for test files (i.e. **/*.spec.ts). The use of function over arrow syntax actually came up when writing tests in one time in Mocha (see Add tests for SocketModeReceiver (#750) #1012 (review)). As such, we shouldn't have the linter recommend to use only arrow functions in test files, as that may lead to the same situation as in Add tests for SocketModeReceiver (#750) #1012.
• Loosens up how we do type assertions in code (via the consistent-type-assertions rule), to allow for the const x = { ... } as T over the const x: T = { ... } syntax. The latter requires all properties of a type to be declared, whereas the former requires only the required properties to be defined. I believe the as T syntax is handy to have in test code.
• Turns off the symbol-descriptor rule and allows for creating Symbol objects without a descriptor.

Impact

Before

✖ 362 problems (173 errors, 189 warnings)
  76 errors and 0 warnings potentially fixable with the `--fix` option.

After

✖ 330 problems (141 errors, 189 warnings)
  64 errors and 0 warnings potentially fixable with the `--fix` option.

Requirements (place an x in each [ ])

• I've read and understood the Contributing Guidelines and have done my best effort to follow them.
• I've read and agree to the Code of Conduct.